Ask Cassie!

But why does being far away matter?

Imagine looking out the car window. A mailbox beside the road zooms past you. A faraway mountain seems to move much more slowly.

The Moon is about 239,000 miles away, so even when you move quite a long way, your viewing angle to the Moon changes only a tiny amount. Meanwhile, nearby trees, houses, and signs move quickly through your view.

Your brain compares the Moon to those nearby objects, making it seem like the Moon is traveling right along with you.

Doesn't the Moon only come out at night?

Nope! The Moon travels around Earth, so it rises and sets at different times on different days.

The Moon doesn't make its own light. Sunlight shines on the Moon and reflects back toward us. Sometimes the Moon is in the daytime part of our sky, and enough reflected sunlight reaches our eyes for us to see it even though the sky is bright.

So what's happening?

The Sun always lights up half of the Moon, just like shining a flashlight on a ball lights one side.

But the Moon travels around Earth. As its position changes, we see different amounts of the bright side.

Sometimes we see almost none of it. Sometimes we see a little crescent. Sometimes we see half. And sometimes the whole side facing us is illuminated.

But how can air make light twinkle?

Earth's atmosphere contains layers of air with slightly different temperatures and densities. Starlight bends a tiny bit as it travels through those constantly moving layers.

Stars are so far away that they look like tiny points of light to us. Those little changes in the path of their light can make them appear to wiggle, brighten, dim, and even change color for a moment.

Why is daytime so bright?

Sunlight enters Earth's atmosphere and gets scattered by molecules and tiny particles in the air. That scattered sunlight makes the whole sky glow.

Stars are much dimmer in our sky than the Sun, so their light gets lost in all that brightness.

Imagine trying to see a tiny flashlight while a giant stadium light is shining nearby. The little light didn't disappear — it's just much harder to see.

Wait — sunlight has colors?

It does! Sunlight looks white, but it's actually made of all the colors of the rainbow.

Those colors have different wavelengths. Blue light has a shorter wavelength than colors like red and orange.

When sunlight enters Earth's atmosphere, it interacts with tiny molecules in the air. Shorter wavelengths — especially blue — are scattered much more strongly than longer red wavelengths. That means blue light reaches your eyes from all across the sky.

Why does that change the colors?

During the middle of the day, sunlight takes a relatively short path through the atmosphere. But when the Sun is near the horizon, its light travels through a much longer stretch of air.

Along that longer journey, much of the shorter-wavelength blue light gets scattered away from the direct path. More of the longer-wavelength reds and oranges make it through to your eyes.

Dust, pollution, smoke, and tiny particles in the atmosphere can change the colors even more.

How can water separate colors?

White sunlight contains many colors. When sunlight enters a raindrop, the light bends. Different colors bend by slightly different amounts.

The light reflects off the inside of the droplet and bends again as it comes back out. That separates the sunlight into the colors we see in a rainbow.

How does that change my shadow?

Light travels mostly in straight lines. When your body blocks sunlight, the area behind you doesn't receive that direct light. That's your shadow.

When the Sun is high in the sky, its light shines down at a steep angle and your shadow is shorter. When the Sun is low near the horizon, the light hits you from the side and your shadow stretches much farther across the ground.

How does spinning make day and night?

Earth spins all the way around once every 24 hours — that's one full day. When your part of Earth is facing the Sun, sunlight reaches you and it's daytime where you are.

As Earth keeps spinning, your part of Earth slowly turns away from the Sun, and it becomes nighttime for you, even while it's still daytime for someone on the other side of the world!

Lightning heats the air around it incredibly fast. That hot air suddenly expands, creating a powerful sound wave. That's the thunder you hear.

Light travels about 186,000 miles every second, so you see the flash almost instantly. Sound travels through air at only about 760 miles per hour, so the thunder takes longer to reach you.

Where does the water come from?

The air you breathe out contains invisible water vapor.

When that warm vapor quickly cools outside your mouth, some of it changes into teeny-tiny liquid droplets. Those droplets scatter light, which makes the little cloud visible.

But why are there different pressures?

The Sun heats Earth's surface unevenly. Land, water, forests, cities, and other surfaces don't all warm at the same rate.

Warm air expands and becomes less dense, so it tends to rise. That can create lower pressure near the ground. Cooler, denser air nearby moves toward the lower-pressure area. That moving air is wind.

Why don't those droplets fall?

A cloud isn't like a giant bucket of water hanging in the sky.

Each individual droplet is extremely small and falls very slowly. Air currents, including rising air, can help keep those tiny droplets suspended.

As droplets collide and join together, they can become larger and heavier. Eventually, the air can no longer keep them suspended. Then they fall.

Where did that water come from?

The Sun warms water in oceans, lakes, rivers, soil, and other places. Some becomes invisible water vapor and rises into the atmosphere.

As that air rises and cools, water vapor condenses onto tiny particles in the air, forming little droplets. Those droplets can collide and join together. Eventually, some become large enough to fall as rain.

Why does that make it white?

When light enters a pile of snow, it hits all those tiny ice surfaces and gets reflected and scattered again and again.

Visible sunlight contains all the colors of the rainbow. Snow scatters those colors fairly evenly back toward our eyes. When all those colors reach our eyes together, we see white.

How can liquid water become a gas?

Water molecules are always moving.

Some molecules near the surface of a puddle gain enough energy to escape from the liquid and enter the air as water vapor. Warmth, wind, and dry air can make this happen faster.

But what does density mean?

Density tells us how much matter is packed into a certain amount of space.

Imagine putting 10 toy dinosaurs into a tiny box. Now put the same 10 dinosaurs into a bigger box and spread them out. You have the same number of dinosaurs, but they're less tightly packed in the bigger box.

Water does something unusual when it freezes — its molecules arrange themselves into a structure that leaves more empty space between them. So the same amount of frozen water takes up more space than it did as a liquid, which makes ice less dense than liquid water, so it floats.

How does salt get from rocks into the sea?

Rainwater is slightly acidic and slowly breaks down rocks. Water carries dissolved minerals and ions into streams and rivers, which eventually flow into the ocean.

The ocean also receives dissolved materials from places on the seafloor, including hydrothermal vents. Water constantly leaves the ocean through evaporation — but the dissolved salts don't evaporate with it.

Is the water traveling all the way to shore?

Not exactly! As a wave moves across deep water, it's mostly the energy traveling forward. Individual water particles mostly move in small circular or looping motions.

Near shore, the ocean becomes shallower. The bottom of the wave interacts with the seafloor, slowing it down. The wave gets steeper until the top tips forward and breaks.

What's chlorophyll?

Chlorophyll is a green pigment that helps plants capture light energy for photosynthesis.

As days get shorter and temperatures change, many trees prepare for winter. They stop making as much chlorophyll, and the green color begins to disappear.

That reveals yellow and orange pigments called carotenoids. Some trees also make red and purple pigments called anthocyanins.

But how does water get all the way up into the sky?

Water from oceans, lakes, rivers, plants, and even puddles can enter the air as invisible water vapor. Warm, moist air can rise higher into the atmosphere, and as it rises, it usually cools.

Cooler air can't keep as much water vapor in gas form. Some of that water vapor changes back into tiny drops of liquid water or, when it's cold enough, ice crystals. Those tiny droplets and crystals gather around microscopic particles in the air and together form a cloud.

Wait — my body is doing it on purpose?

Yes! Scientists once thought wrinkly fingers simply happened because skin soaked up water.

But water wrinkles are controlled by nerves. When the blood vessels in your fingertips narrow, the tissue underneath takes up slightly less space, causing the skin above it to form wrinkles.

Scientists think those wrinkles may help us grip wet objects, although researchers are still studying exactly why the response evolved.

Why would my body do that?

When you're cold, your nervous system can tell these little muscles to tighten. Each one pulls a hair upward and makes the skin around it rise into a bump.

For furry animals, raised hair traps extra air close to the skin, which helps keep them warm. Humans don't have enough body hair for it to work very well — but we still have the response. Strong emotions can trigger it too.

What's my diaphragm?

It's a large muscle underneath your lungs that plays a big part in breathing. Normally, it moves smoothly down and up as you breathe.

During a hiccup, your diaphragm suddenly contracts. That pulls air quickly toward your lungs. Almost immediately, your vocal cords come together and interrupt that rushing air. HIC! That's the sound you hear.

How does my nose know?

The inside of your nose contains sensitive nerve endings. If something like dust, pollen, or pepper irritates them, those nerves send a message to your brain.

Your brain coordinates muscles in your chest, throat, and face. You take a breath, pressure builds, and air blasts out through your nose and mouth. ACHOO!

What do we know?

Yawning often happens when we're tired, waking up, bored, or changing from one level of alertness to another.

Scientists have proposed several ideas about what yawning does, including helping regulate brain temperature or helping the brain shift its state of alertness. But there isn't one explanation that answers everything yet.

And the old idea that we yawn simply because our bodies need more oxygen? Experiments haven't supported that as the main explanation.

Why is it louder when I'm hungry?

Your digestive tract uses waves of muscle contractions to move its contents along.

When your stomach and small intestine have been fairly empty for a while, they go through cycles of stronger contractions that help sweep leftover material along.

With less food inside to muffle the sounds, the movement of gas and liquid can be easier to hear.

Why does prediction matter?

When you move your own hand, your brain doesn't just tell your muscles what to do. It also predicts what sensations that movement should cause.

So when your fingers touch your own belly or foot, your brain already knows approximately where, when, and how the touch will happen. That makes the sensation less surprising — and tickling depends a lot on unpredictability.

Why does everyone have different ones?

Your fingerprint patterns begin developing before you're born.

Genes help guide their overall development, but tiny differences in growth and conditions inside the womb influence exactly how the ridges form. That's why even identical twins don't have identical fingerprints.

What was the cord for?

Before birth, a baby gets oxygen and nutrients through a special organ called the placenta. The umbilical cord connects the baby to the placenta, and blood vessels inside the cord carry substances between the baby and placenta.

After the baby is born, the cord isn't needed anymore. It is clamped and cut, and the small piece left attached eventually dries up and falls off. The healed spot becomes your belly button.

But what makes the tooth get wiggly?

A baby tooth isn't just the little white part you can see. It originally has roots holding it in your jaw.

As a permanent tooth develops underneath, special cells gradually break down much of the baby tooth's root. With less root holding it in place, the baby tooth becomes loose. Wiggle, wiggle, wiggle… and eventually out it comes!

How does my body make one?

When a blood vessel is damaged, tiny pieces in your blood called platelets gather at the injury. Proteins in your blood help form a mesh called fibrin, which traps blood cells and strengthens the clot.

As the clot at the surface dries, it forms a scab. Underneath that protective covering, new skin cells can grow and repair the wound.

Is my brain asleep too?

Your brain never completely switches off. During sleep, different brain areas remain active. Dreams can include memories, feelings, people, places, and completely imaginary things.

Dreaming can happen during different stages of sleep, but especially vivid dreams are common during REM sleep, when brain activity is high and your eyes move rapidly behind your eyelids.

Scientists are studying whether dreaming is connected to things like memory, emotions, learning, and how our brains process experiences.

There's fluid in my ears?

Yep! Deep inside each ear are three curved, fluid-filled tubes called semicircular canals.

When you turn your head, the fluid moves and bends tiny sensors. Those sensors send information to your brain about how your head is moving.

Spin around and the fluid starts swirling. Stop suddenly, and the fluid keeps moving briefly. Your eyes say, “We've stopped!” while your inner ears are still sending signals that suggest movement. Your brain gets conflicting information — and you feel dizzy.

What are the two ways?

Some sound travels through the air from your mouth to your ears — just like when you listen to another person.

But vibrations from your voice also travel through the bones and tissues inside your head to your inner ear. Those internal vibrations make your voice sound different to you, often fuller or deeper.

A recording captures mainly the sound that traveled through the air.

What are germs?

“Germs” is a simple word we use for tiny things that can sometimes cause illness. Two important kinds are viruses and bacteria.

Viruses can enter our cells and use them to make more viruses. Some bacteria can also multiply in our bodies and cause illness.

Luckily, your body has a whole defense system designed to find things that shouldn't be there and fight them — that's your immune system. Sometimes things you feel when you're sick, like a fever, stuffy nose, or cough, are partly caused by your immune system working to protect you.

And not every sickness comes from germs. Allergies, injuries, genetics, and other things can make our bodies feel unwell too.

But how do germs get on my hands?

Your hands touch things all day long — doorknobs, toys, tables, playground equipment, pets, food, and other people. Some of those surfaces can have microbes on them.

Then you might touch your eyes, nose, or mouth, giving certain germs a way into your body. You can also move germs from your hands onto other people or objects.

So what does soap actually do? Water alone can rinse some things away, but soap makes handwashing much more effective. Soap molecules have a clever structure: one end interacts well with water, while another interacts with oils and grease. That helps loosen oils, dirt, and microbes from your skin so they can be swept away when you rinse. Soap can also disrupt the fatty outer layer of some viruses.

Wait — what is pee actually made of?

Your blood is constantly traveling through your kidneys. Your kidneys are amazing filters — they keep things your body needs while helping remove extra water and certain wastes from your blood.

Those wastes and extra water become urine, or pee, and travel to your bladder until you're ready to go to the bathroom. Pee gets its yellow color from a special yellow coloring your body makes as it recycles old blood cells.

Why is it sometimes really light yellow and sometimes darker? The amount of water in your urine makes a big difference. When there's lots of water mixed in, your pee is more diluted and usually looks pale yellow. When there's less water, the yellow pigment is more concentrated, so your pee can look darker.

Aren't tears just for keeping my eyes clean?

You actually make tears all the time to keep your eyes clean and comfortable — those are always quietly at work. But strong feelings, like sadness or even really big happiness, can trigger a different kind of tears made just for emotions.

Scientists think these emotional tears may help your body release stress, and they can also let people around you know you might need some comfort.

What's happening to my hair?

Atoms contain negatively charged particles called electrons. When two materials rub together, electrons can move from one material to the other.

After you rub a balloon on your hair, many hairs can end up with the same type of electric charge. Charges that are alike push away from each other. So your individual hairs push apart and stand up.

Why those things?

Atoms have magnetic properties connected to their electrons. In some materials, groups of atoms can line up so their magnetic effects work together.

A nearby magnet can cause many of these magnetic regions to line up, creating a strong attraction. In materials such as wood or plastic, the particles don't line up in the same useful way, so you don't get that strong magnetic pull.

Why does the bubble care about that?

A bubble has a very thin film of soapy water surrounding air. The molecules in that film pull on one another, creating surface tension, which tries to make the surface as small as possible.

For a bubble floating freely in the air, the smallest possible surface for the amount of air inside is a sphere.

So why can a giant ship float?

When you put something in water, it pushes some water out of the way. We call that displacement. The water pushes upward on the object with a force called buoyancy.

If the upward buoyant force can balance the object's weight, it floats.

That's why shape matters. A solid chunk of steel is very dense and sinks. But a steel ship is mostly hollow space filled with air. Its mass is spread over a much larger volume, so its average density can be low enough to float.

But how can air hold up something so heavy?

Air may be invisible, but it's made of molecules and can exert forces. The engines provide thrust, pushing the airplane forward through the air.

As air flows around the specially designed wings, the wings turn some of that airflow downward and create pressure differences around the wing. Together, these effects produce an upward force. That's lift.

When lift is great enough to balance or overcome the airplane's weight, the airplane can rise or stay in the air.

But how can something THAT heavy float?

If you dropped a solid block of steel into the ocean, it would sink. But a cruise ship isn't a giant solid block of steel — most of the space inside its huge hull is filled with air.

The ship's wide, hollow shape spreads its mass over a very large volume. That makes the average density of the whole ship — including all that air — less than the density of the water it floats in.

As the ship settles into the water, its hull pushes water out of the way. The more the ship sinks down, the more water it pushes aside. When the weight of the water being pushed aside equals the weight of the ship, the upward buoyant force balances the ship's weight — and the ship floats!

But how does that actually happen?

Older-style bulbs have a tiny thin wire inside. Electricity flows through that wire and makes it so hot that it glows brightly, almost like a sparkler!

Many bulbs today are LEDs instead. Inside an LED, electricity moves through tiny special parts that light up all on their own, without needing to get super hot first. That's part of why LED bulbs stay much cooler to the touch.

What's the journey?

Electricity is generated when energy from sources such as wind, sunlight, flowing water, nuclear reactions, or fuels is converted into electrical energy.

For much of the grid, transformers raise the voltage so electrical energy can be transmitted efficiently over long distances through power lines. Near towns and neighborhoods, substations and transformers lower the voltage in stages.

Finally, wires carry electricity into your home, where circuits deliver it to lights and outlets.

So what happens when I flush?

When you push the handle or button, water stored in the tank — or supplied by another flushing system — rushes into the toilet bowl. The toilet has a curved passage called a trapway.

As water rapidly fills and moves through that curved passage, it can start a siphoning action that pulls the bowl's water and waste through the trapway and into the drainpipe. Fresh water then refills the bowl.

From there, household pipes usually carry the wastewater to a sewer system or septic system.

How do you move heat?

A refrigerator circulates a special substance called a refrigerant through a closed system. Inside the refrigerator, the refrigerant can evaporate at low pressure. As it changes into a gas, it absorbs heat from the air and food around it.

A compressor then squeezes the refrigerant gas, raising its pressure and temperature. The hot refrigerant travels through coils outside the cold compartment and releases heat into the room.

It condenses back toward a liquid, its pressure is reduced, and the cycle starts again.

Wait — air is pushing on my drink?

It is! Earth's atmosphere is pressing on everything around you, including the top of your drink.

When you suck air out of a straw, you reduce the pressure inside the straw. Now the air pressure pushing down on the surface of the drink is greater than the pressure inside the straw.

That pressure difference pushes liquid up the straw and toward your mouth.

How can invisible waves make food hot?

Almost all food has water hiding inside it, even food that seems dry. The waves from a microwave reach inside the food and make that water jiggle back and forth incredibly fast.

All that fast jiggling creates heat — kind of like how rubbing your hands together quickly makes them feel warm. That heat spreads through your food until it's warmed all the way through.

How can something in space find me?

High above Earth, there are satellites — a little like tiny spaceships — that are always sending out signals.

Your phone listens for signals from several of these satellites at the same time. By comparing them, your phone can figure out exactly where you are on Earth — kind of like how you could figure out where you're standing by spotting several landmarks around you at once.

What's the counterweight for?

Imagine a seesaw. Instead of making the motor lift the entire weight of the elevator car and its passengers all by itself, many elevator systems have a heavy counterweight connected to the other side.

As the elevator goes up, the counterweight goes down — and vice versa. The motor turns a pulley called a sheave, moving the ropes or belts attached to the car and counterweight.

Elevators also have multiple braking and safety systems.

Why would my eyes need help focusing?

For you to see a sharp image, light entering your eye needs to be focused onto the retina at the back of your eye. Sometimes the shape of the eye or its focusing system causes light to focus in the wrong place.

For example, in nearsightedness, distant objects look blurry because light tends to focus in front of the retina rather than directly on it.

A carefully shaped glasses lens bends the light before it enters the eye, helping move the focus to the right place. Different vision problems need lenses that bend light in different ways.

Why does that make it float?

Air is made of tiny invisible bits, and so is helium — but helium's bits are much lighter and floatier than the bits that make up regular air.

When you fill a balloon with helium, the whole balloon becomes lighter than the air around it, so it gets pushed upward and floats, almost like a bubble rising up through water.

A balloon filled with your own breath doesn't float, because your breath is just regular air, which isn't light enough to rise on its own.

What does fire actually need to happen?

Fire needs three things to get going: something that can burn, like wood or paper; enough heat to get it started; and oxygen from the air around it.

When those three things come together, the material breaks apart in a fast reaction that releases a lot of heat and light. The flame you see is actually hot glowing gas, rising up and shining brightly.

There's water inside popcorn?!

Yep! A popcorn kernel contains a small amount of water inside a hard outer shell.

When the kernel gets hot, that water turns into steam. The steam takes up more space and pushes against the shell. Pressure keeps building until the shell can't hold it anymore. POP! The shell bursts, and the hot, soft starch inside quickly expands and cools into the fluffy popcorn you eat.

Why was gas inside my drink?

When soda is made, carbon dioxide gas is forced into the liquid under high pressure. The sealed bottle or can helps keep the gas dissolved in the drink.

When you open it — pssssst! — the pressure above the soda suddenly drops. Now the carbon dioxide can leave the liquid more easily, forming bubbles that rise to the surface and escape into the air.

Why does the onion do that?

An onion stores different chemicals and enzymes inside its cells. When you cut it, you break those cells open and allow substances that were kept apart to mix.

Chemical reactions then produce an irritating compound that can travel through the air. When it reaches your eyes, your nerves notice the irritation and tell your tear glands to make tears to help wash it away.

Why doesn't it happen before I cut it?

An apple's skin and intact cells keep certain substances separated from the oxygen in the air.

Cutting or bruising the apple damages those cells. An enzyme in the apple can then help reactions happen that eventually create brown-colored compounds.

What's happening inside the toaster?

When bread gets hot enough, certain sugars and amino acids begin a whole group of chemical reactions.

Those reactions create hundreds of new compounds. Some make toast brown. Others give toasted bread its delicious smell and flavor.

But where does the heat come from?

Heat naturally transfers from warmer objects to cooler ones. The air, your hand, and even the bowl are usually warmer than your ice cream.

Energy moves from those warmer surroundings into the ice cream. As the frozen water in the ice cream absorbs enough energy, its molecules can move more freely and the ice turns into liquid water.

Where does the gas come from?

In many breads, it comes from yeast. Yeast is a living microorganism. When yeast has access to certain sugars, it can break them down and release carbon dioxide gas through a process called fermentation.

The dough contains stretchy networks of proteins — especially gluten in wheat dough — that help trap those gas bubbles. As more gas forms, the bubbles expand and the dough gets bigger and puffier.

Why don't all cheeses melt the same way?

Cheese is a mixture of water, fat, proteins, salt, and other substances. Its proteins form a network that helps hold everything together.

As cheese warms, its fat melts and the protein structure becomes more flexible. In cheeses with the right balance of moisture, fat, acidity, and protein structure, the cheese can become soft, stretchy, or gooey.

Different cheeses have different amounts and arrangements of those ingredients, so they behave differently when heated.

Why does chocolate melt so easily?

Chocolate contains a fat called cocoa butter. The molecules in solid cocoa butter are arranged into crystal structures. When they absorb enough heat, those structures begin breaking apart and the fat becomes liquid.

Well-made chocolate is often prepared so it stays solid at a comfortable room temperature but begins melting around body temperature. Your hand is warm enough to transfer heat into the chocolate.

Why does clear egg white turn white?

Raw egg white contains lots of proteins folded into particular shapes and surrounded by water. Heating gives those protein molecules more energy, and they begin to unfold.

Once unfolded, they can bump into one another and form new connections, creating a network that traps water. That new structure scatters light differently, so the clear, runny egg white becomes white and firm.

What's happening inside the noodle?

Dry pasta contains lots of starch packed into a firm structure. As pasta sits in hot water, water enters it. The starch granules absorb water and swell, and heat changes how the starch molecules are organized.

The pasta becomes more flexible and tender. Keep cooking it for too long, though, and its structure continues weakening — which is how you end up with mushy noodles.

Why don't they mix?

Water molecules are polar, meaning their electrical charges are unevenly distributed. That makes water molecules strongly attracted to other water molecules.

Oil molecules are mostly nonpolar. They don't interact with water in the same way, so the water tends to stick with water while oil sticks with oil. The two liquids separate.

Then density determines which layer goes on top. Most cooking oils are less dense than water, so the oil floats.

How does salt do that?

Plain water turns into ice once it gets cold enough. But when salt mixes into water, it gets in the way of the water forming its neat, frozen ice pattern.

That means salty water has to get even colder before it can freeze solid. So when you sprinkle salt onto ice that's already there, the salt mixes in and the ice starts turning back into liquid water, even though the air outside is still chilly.

What's happening to it?

Fruit gets much of its firmness from the walls and other structures around its cells. Even after fruit is picked, natural enzymes inside it continue causing chemical changes.

Cutting damages cells and removes the protection of the intact skin. Over time, substances that help hold the cell walls together can break down, water can move out of damaged cells, and microorganisms can eventually begin growing too.

All of that can make the fruit softer and mushier.

Why does heat make them escape?

Smells come from molecules that can leave food and travel through the air. When food gets warmer, its molecules generally have more kinetic energy, so more aroma molecules can escape from the food into the air.

Those airborne molecules enter your nose and bind to smell receptors, which send signals to your brain. More aroma molecules reaching your nose can mean a stronger smell.

How can food trick my nerves?

Chili peppers contain a chemical called capsaicin. Capsaicin can attach to a type of receptor on certain sensory nerve cells — receptors that also respond to potentially harmful heat.

When capsaicin activates them, the nerves send a warning signal to your brain. Your brain interprets that signal as “HOT!” even if the pepper itself isn't physically hot.

Why does my forehead hurt when my mouth got cold?

The roof of your mouth is full of nerves, and so is your forehead. When something very cold touches the roof of your mouth, nearby blood vessels quickly shrink and then open back up again.

That sudden change can confuse your nerves, which send a pain signal racing to your brain. Your brain isn't quite sure exactly where that signal came from, so it feels like the pain is coming from your forehead instead of your mouth.

How do taste buds actually work?

Your tongue is covered with thousands of tiny taste buds, and each one has special cells that can sense different flavors.

When you eat, little bits of food mix with your spit and touch your taste buds. Your taste buds send a signal racing to your brain, and your brain quickly figures out what you're tasting.

Your nose helps too — that's part of why food can taste bland when you have a stuffy nose!

Why does that matter?

Electric current flows through a body when there is a difference in electric potential between two points and a conductive path connects them.

If both of a bird's feet are close together on the same wire, they're usually at almost the same voltage.

But if the bird touched two wires at different voltages, or a wire and something connected to the ground, electricity could flow through its body — and it could be badly hurt.

How does it work while they're asleep?

Tendons are strong tissues that connect muscles to bones. In many perching birds, bending the legs while sitting on a branch pulls on tendons that curl the toes around the perch.

Their own body position helps maintain the grip, so they don't have to consciously squeeze the branch all night.

Wait — there's oxygen gas in water?

Yes! Oxygen from the atmosphere can dissolve into water, and aquatic plants and algae also release oxygen during photosynthesis.

A fish moves water across its gills. The gills contain lots of thin surfaces with tiny blood vessels. Dissolved oxygen moves from the water across those surfaces into the fish's blood, while carbon dioxide moves from the blood into the water.

But penguins have wings!

They do — but penguin wings are very different from the wings of flying birds.

Over millions of years, penguin ancestors adapted to hunting underwater. Their wings became shorter, flatter, and stiffer, making them powerful flippers. Their bodies and bones also became well suited to diving and swimming.

Those adaptations make penguins amazing underwater — but too specialized for flying through the air.

What actually happens?

A butterfly starts life as an egg. A caterpillar — also called a larva — hatches from that egg. Its big job is to eat and grow. As it gets bigger, it sheds its outer covering several times.

Eventually, the caterpillar forms a protective casing called a chrysalis. And this is where something incredible happens: inside the chrysalis, the caterpillar's body goes through a huge reorganization. Some tissues are broken down and recycled, while groups of cells that were already inside the caterpillar grow and develop into adult butterfly structures — like wings, long legs, antennae, and new mouthparts.

When that transformation is complete, the adult butterfly emerges.

But are their eyes actually making light?

Nope! A cat's eyes aren't little flashlights. Light enters through the pupil and reaches the retina, the light-sensitive layer at the back of the eye.

Behind the retina, cats have a shiny layer called the tapetum lucidum. Light that passes through the retina can bounce off this layer and travel through the retina again. That helps cats use small amounts of light very efficiently — which is useful for seeing when it's dim.

Some of that reflected light also comes back out of the eye toward you. That's the “glow” you see.

Why would a wet nose be useful?

Dogs experience a huge part of their world through smell. When odor molecules float through the air, some can dissolve in the moisture covering a dog's nose. That can help those smells reach the dog's scent-detecting system.

Dogs also lick their noses, which helps keep them moist and can move scent molecules toward another smell-sensing organ in the mouth.

Wait — they can sense Earth's magnetic field?

Some birds can! Earth acts a little like a giant magnet. Evidence shows that many migratory birds can detect information from Earth's magnetic field and use it as part of their navigation system.

But that's not their only clue. Depending on the species, birds can learn landmarks, follow coastlines, use the position of the Sun during the day or stars at night, detect smells, and learn routes from experienced birds.

Some parts of migratory behavior are inherited, while other parts are learned.

Are they trying to match whatever they're sitting on?

Not usually! A chameleon's skin has special see-through layers with tiny structures inside them. When the chameleon relaxes or tightens its skin, the spacing of those tiny structures changes.

That change in spacing changes which color of light bounces back to your eyes — and that's the color you see! Chameleons often change color because of temperature, mood, or to "talk" to other chameleons.

Blending in can happen too, but a chameleon isn't simply looking at a leaf or a rock and copying its color.

Can they really see when there's NO light?

No. Eyes need light to see. But nighttime usually isn't completely dark — there's light from the Moon, stars, sky, or nearby human-made sources.

Many nocturnal animals have large pupils that let more light into their eyes. Their retinas may also contain lots of light-sensitive cells called rods, which work especially well in dim conditions.

Some animals also have a reflective tapetum lucidum that gives incoming light another chance to reach the retina.

How can a behavior be inherited?

An animal's brain and body come already a little bit "programmed" before it's even born, so some behaviors are ready to go without anyone teaching them. A newly hatched spider, for example, doesn't need to attend web-building class!

We often call behaviors like this innate behaviors. But animals aren't born knowing everything — many also learn by practicing, watching others, or being taught by their parents.

Do they just sleep all winter?

It's more complicated than ordinary sleep. During true hibernation, an animal's metabolism slows dramatically. Its body temperature may drop, and its heart rate and breathing can become much slower.

That means the animal needs much less energy to stay alive.

And not every animal we casually call a “hibernator” does the same thing. Bears, for example, enter a winter state with important differences from the deep hibernation of some small mammals.

But there's no glue on their feet!

Right — geckos don't need sticky slime. Each tiny hair branches into even tinier hairs, so a gecko's foot touches way more of the wall than you'd ever guess.

When those tiny hairs get super close to the wall, a very gentle, natural stickiness between the surfaces starts to pull them together. Scientists call this pull van der Waals forces — try saying that three times fast!

One tiny pull by itself is way too weak to notice. But millions of them happening together add up to enough force to hold the whole gecko up.

How could stripes bother a fly?

Scientists have compared zebras with other animals and even conducted experiments using striped coverings. Biting flies seem to have more trouble successfully landing on striped surfaces.

Researchers have considered other possibilities too, including camouflage, temperature control, and social recognition. Science continues to investigate exactly how stripes may help zebras.

How does that become a web?

Spider silk begins as special proteins stored inside silk glands. When a spider needs silk, the material travels through its spinnerets and becomes a strong thread.

A web-building spider can attach one thread, stretch it to another spot, and gradually build a framework, then add more carefully arranged strands.

Different silk can do different jobs. Some is strong for supporting the web, some can be sticky for catching prey, and some can wrap eggs or protect the spider.

So a flower isn't just there to look pretty?

Nope! A flower has an important job for the plant. Many flowers contain structures that make pollen and structures that can eventually develop seeds.

For a seed to form, pollen needs to reach the right part of a flower — this is called pollination. Some plants use wind to move pollen. Others get help from animals such as bees, butterflies, birds, and even bats. That's why many flowers have bright colors, interesting shapes, smells, or sweet nectar — they can help attract the animals that carry pollen from place to place.

But how does a plant know when to bloom? Plants can sense what's happening around them. Depending on the species, changes in day length, temperature, rainfall, and other conditions can help tell a plant when it's a good time to flower.

But how does it happen?

A seed contains three important things: a tiny developing plant called an embryo, stored food to help it get started, and a protective seed coat around the outside.

When the seed absorbs water, it swells and its cells become active. The embryo begins using its stored food for energy.

Usually, a tiny root comes out first and grows into the soil. Then a shoot grows upward. Once leaves open and reach the light, the young plant can begin making much more of its own food through photosynthesis.

How can a plant make food?

A plant takes in water through its roots and carbon dioxide gas from the air through tiny openings, mostly in its leaves.

Inside the leaves is a green pigment called chlorophyll that absorbs light energy. The plant uses that energy to rearrange water and carbon dioxide into a type of sugar called glucose, which it can use for energy and to build new plant material.

Oxygen is produced along the way and released into the air.

But how does the water get inside?

Roots have thousands of tiny structures called root hairs that greatly increase the area touching the soil. Water moves from the soil into the root tissues, then enters tiny tubes inside the plant called xylem.

The xylem carries water upward through the stem and into the leaves.

Plants don't “drink” by sucking water up like a straw. Water movement is driven largely by evaporation from the leaves, along with the way water molecules stick to one another and to the walls of the xylem.

What does sunlight actually do?

Think about your body needing energy to run, play, grow, and repair itself. Plants need energy too.

Green chlorophyll inside plant cells absorbs light energy. During photosynthesis, the plant uses that energy to turn carbon dioxide and water into glucose, a sugar that stores chemical energy.

The plant can then use those sugars to grow stems, roots, leaves, flowers, and fruit.

But why does chlorophyll look green?

Sunlight contains many different colors. Chlorophyll absorbs certain wavelengths especially well, including lots of red and blue light — but it doesn't absorb green light as strongly.

More green light is reflected or transmitted, and some of that light reaches your eyes. That's why the leaf looks green!

How does that happen?

Every flower has a part that makes pollen and a part where new seeds can start growing. When pollen reaches that part, it helps the flower begin making seeds.

Once that happens, the flower usually doesn't need its petals anymore, so they fall away. The base of the flower stays behind and slowly grows bigger and juicier, changing into a fruit that wraps around the new seeds inside.

That's why you can sometimes spot a little shriveled-up flower still stuck to the end of a tiny growing fruit!

What is it waiting for?

Many seeds stay in a resting state called dormancy until conditions are suitable.

Water is one of the most important signals. When a seed absorbs enough water, chemical reactions and enzymes inside it become active. Temperature, oxygen, and sometimes light or darkness can also affect whether particular seeds germinate.

Different plants have different requirements. Some seeds even need a period of cold — or exposure to things like fire or smoke — before they can germinate well.

How can a plant sense gravity?

Certain cells near the tips of roots contain tiny, heavy bits inside them that settle toward the bottom of the cell, the same way sand settles to the bottom of a jar of water. That helps the plant sense which direction is down.

The plant then grows a little more on one side of the root than the other, which slowly curves the root downward, toward the ground.

Shoots respond differently, generally growing upward and toward the light.

How can leaves pull water upward?

Tiny openings in leaves let water vapor escape into the air — this is called transpiration. As water leaves, it helps create tension that pulls on the water molecules behind it inside the xylem.

Water molecules also tend to stick to one another. So when molecules near the top are pulled upward, they help pull other water molecules along — almost like a long, connected chain. Water also interacts with the walls of the narrow xylem tubes.

Together, these processes help move water from roots to leaves, even in very tall trees.

Are the bees trying to help?

Usually, they're looking for food! Flowers can provide bees with sugary nectar and protein-rich pollen.

As a bee crawls around a flower, pollen grains stick to the tiny hairs on its body. When it visits another flower of the same kind, some of that pollen can rub off and help that flower start growing seeds.

That can lead to new seeds and fruit growing on the plant.

How does that work?

Leaves have tiny openings called stomata. Carbon dioxide can enter through these openings and be used during photosynthesis, and oxygen produced during photosynthesis can leave through them.

But plants also perform cellular respiration, just like animals do. Their cells use oxygen to help release usable energy from sugars, producing carbon dioxide and water.

Plants respire day and night. Photosynthesis requires light, so that part happens only when enough light is available.

What makes a cactus so good at saving water?

Many cacti have thick, fleshy stems that can store water, and a waxy outer covering that helps slow water loss.

Instead of having big, thin leaves, most cacti have spines — tiny spines lose much less water than broad leaves would. Many cacti also open their gas-exchange pores mostly at night, when temperatures are cooler, helping them conserve even more water.

Their roots can quickly absorb water when rain finally arrives.

But plants don't have brains!

They don't need one to respond to their environment. When something bends a trigger hair, cells in the plant create an electrical signal.

The trap usually needs more than one stimulation within a short time before it snaps shut. That helps prevent the plant from wasting energy by closing every time a raindrop or piece of dirt touches it.

Once the right signals occur, the curved leaves rapidly change shape and snap together. If prey continues moving inside, more signals help trigger the plant's digestive response.

What kind of evidence?

When an organism dies, it usually disappears completely. But under special conditions, parts of it — or traces it left behind — can be preserved for millions of years as fossils.

Scientists have discovered fossilized dinosaur bones, teeth, footprints, eggs, nests, skin impressions, and feathers.

They can study where fossils were found, compare skeletons, examine rocks around them, and use scientific dating methods to determine how old those rocks are. All those clues fit together.

But fossils are just bones, right?

Not always! Skeletons tell us a lot about a dinosaur's size, posture, joints, and proportions. Places where muscles attached to bones can give scientists clues about muscles.

And some exceptional fossils preserve skin impressions, feathers, body outlines, and other soft tissues or traces.

Scientists also compare dinosaur anatomy with living relatives — especially birds and crocodilians.

How can color become a fossil?

Some very well-preserved fossils have tiny color-making dots left inside them, so small you'd need a powerful microscope to see them. Living animals have these same tiny dots in their feathers, skin, and fur, and they help make colors.

Different shapes of these tiny dots match up with different colors in animals alive today. So scientists can look at the shapes left behind in a dinosaur fossil and compare them to animals we know now, to make a careful, evidence-based guess about what color that dinosaur might have been.

Were they flying?

Not necessarily. Feathers existed before modern bird flight. Early feathers may have helped dinosaurs stay warm, display colors or patterns, protect eggs, or communicate.

Some dinosaurs later evolved feathers that helped with gliding or powered flight. Feathers were especially common among certain theropod dinosaurs, the group that also gave rise to birds.

How can scientists possibly know that?

Teeth give us big clues. Sharp, serrated teeth are good for cutting flesh. Broad or ridged teeth can help crush or grind plants.

Jaw shape, bite marks, fossilized poop, stomach contents, and even microscopic wear patterns on teeth can provide more evidence. Scientists combine all those clues rather than relying on just one.

Why can't scientists tell?

Sounds don't turn into fossils, and the soft parts of the body that make sounds usually don't survive that long either.

So scientists study dinosaur skulls and bones, and compare dinosaurs to their closest living relatives today: birds and crocodiles. Those animals make all kinds of sounds — booms, hisses, honks, coos, and deep rumbly calls.

Some dinosaurs may have made sounds using their throats or mouths, or maybe even special body parts — but the loud movie-style roar is mostly just made up for the movies.

How could we know that?

Scientists have found dinosaur nesting sites containing eggs arranged carefully in nests. Some fossils show adult dinosaurs positioned over nests in ways similar to brooding birds.

We've also found groups of babies and juveniles living around nesting areas. One dinosaur, Maiasaura, became famous partly because fossils from nesting colonies suggested its babies remained in nests and may have received parental care.

But don't we have the bones?

We do! We know T. rex had relatively short arms with two-fingered hands, and those arms were surprisingly muscular for their size.

What we don't know is exactly what T. rex used them for — or why they became so short during evolution. Scientists have proposed several ideas: they might have helped with gripping, getting up, mating, or something else. It's also possible that as the head and jaws became enormous and powerful, long arms simply weren't as important anymore.

But we don't yet have enough evidence to choose one explanation with confidence.

Why can't we just measure one?

Because we rarely find a complete skeleton from the very largest dinosaurs. Scientists may find a leg bone, vertebrae, or part of a skeleton and use those bones to estimate the animal's total size.

Dinosaurs such as Argentinosaurus and Patagotitan are among the largest known candidates, reaching roughly 30 meters or more in length in some estimates and weighing many tens of tons.

But estimates can change when new fossils are discovered or scientists improve their calculations.

Then what was a pterodactyl?

The animals people often call “pterodactyls” belonged to a group of flying reptiles called pterosaurs. Pterosaurs and dinosaurs shared an ancient ancestor, but they belonged to separate branches of the reptile family tree.

Pterosaurs had wings made from skin and other tissues stretched mainly along an enormously lengthened fourth finger. Dinosaurs had their own set of anatomical features that defined their group.

So what was alive with dinosaurs?

Lots of other animals! Dinosaurs shared Earth with insects, fish, amphibians, turtles, crocodilian relatives, pterosaurs, marine reptiles, and small mammals.

But humans came much, much later. Non-bird dinosaurs disappeared about 66 million years ago. Our species, Homo sapiens, has existed for only around 300,000 years.

How could one asteroid do that?

The asteroid was about 6 miles wide — as big as a small mountain — and it slammed into Earth so hard that it caused huge earthquakes, giant waves, and fires. It also blasted a huge cloud of dust and ash high into the sky.

That cloud blocked out sunlight for a long time. Without enough sunlight, plants struggled to grow, and the world turned cold and dark. Many animals couldn't find enough food to survive.

Animals that couldn't handle those big changes died out.

Does the bone just stay there for millions of years?

Sometimes some original material can remain, but fossilization often changes the remains. Imagine a dinosaur dies near a river — its body is quickly covered by mud or sand.

Soft parts usually decay, but harder parts like bones and teeth may remain buried. Over a very long time, groundwater carrying dissolved minerals can move through the buried remains. Minerals can fill tiny spaces in the bone, and chemical changes can alter the original material, while layers of sediment above harden into rock.

Millions of years later, erosion — or a paleontologist — might expose the fossil again.

Wait… a chicken is a dinosaur?!

Scientifically speaking, yes. Dinosaurs formed many different branches over millions of years.

One branch of small, feathered theropod dinosaurs eventually gave rise to birds. When the asteroid impact wiped out all the other dinosaur lineages about 66 million years ago, some early birds survived. Their descendants are the birds living today.

So birds aren't merely related to dinosaurs in the way crocodiles are — birds are part of the dinosaur family tree itself.

Why not? Don't we have dinosaur bones?

Bones alone aren't enough. To bring back an animal, scientists would need a complete instruction book for building its body, called DNA — and that instruction book has to still be in one piece.

The trouble is, DNA slowly falls apart after something dies, kind of like an old piece of paper crumbling over time. Dinosaurs like T. rex lived so long ago that their DNA has had way, way too much time to crumble away completely.

Scientists have found incredible dinosaur fossils, but nobody has ever found a dinosaur's full, readable instruction book.

What about mosquitoes trapped in amber, like in the movies? It's a fun idea, but scientists haven't actually found real, usable dinosaur DNA preserved that way. DNA just can't survive for tens of millions of years.