Why do some objects, such as doors and windows, get bigger and smaller?

Have you noticed that closet doors don’t close as easily in the summer as they do in the winter? It’s because they expand in the heat of the summer and contract during the cold winter. Everything on Earth is made up of tiny particles called molecules, which are in constant motion. When the molecules heat up, they move faster, pulling apart from one another. As they move apart, they take up more space, causing even solid objects to grow slightly larger. Molecules slow down as they cool, and they take up less room. This causes things to shrink a little bit. (Water is an exception. When it freezes, the molecules line up in such a way that the ice takes up more space.)

Why do cats always land on their feet?

Cats owe some of their nine lives to their unique skeletal structure. Cats don’t have a collarbone, and the bones in their spine are more flexible than other animals. This makes it easier from them to bend and rotate their bodies easier during a short fall. A fall of two or more floors, however, can seriously injure a cat. A cat's feet and legs usually can’t absorb the impact of a fall from that distance or higher.

What makes popcorn pop?

A popcorn kernel is actually a seed. At its center is a tiny plant embryo, a life form in its earliest phase. The embryo is surrounded by soft, starchy material that contains water. Surrounding the embryo is a hard shell. When the kernel is heated to about 400 degrees Fahrenheit, the water turns to steam. The pressure from the steam causes the kernel’s shell to explode and the starch to spill out. You have to add the butter!

What causes lightning?

When air rises and falls within a thunderstorm, positive and negative charges form in the cloud. The bottom of the thundercloud has a negative charge, and the top has a positive charge. A flash of lightning happens when a charge becomes so strong that the air can’t stop it from jumping from the cloud to the ground, which has a positive charge. Lightning can also form inside the cloud, moving between the positively and negatively charged areas. The average flash of lightning could turn on a 100-watt light bulb for more than three months. The air near a lightning strike is hotter than the surface of the Sun.

Why do I feel dizzy when I spin?

Inside your ears are tubes filled with liquid. The liquid moves when you move, telling your brain what position your body is in. When you spin, the liquid also spins. The liquid continues to spin after you stop. Your brain thinks you’re still spinning, so you continue to feel that everything is going in circles-until the liquid stops moving.

Why does a knuckleball seem to “dance” toward home plate?

The ball drops and soars unpredictably because it doesn’t spin. The lack of rapid spin turns the seams of the baseball into tiny airfoils—surfaces that create lift and drag when they fly through the air. As the air passes over the seams, tiny swirls are created, causing pockets of low pressure around the surface of the ball. As air rushes in to fill the pockets, the ball is pushed in different directions. If the ball rotates too much, the seams will present a more consistent surface to the wind, and the ball will likewise follow a smoother path.

Why do leaves change color in the fall?

One of the sure signs of fall (besides the beginning of a new school year) is the change in color of leaves from green to bright yellow, orange and red. Trees are sort of like bears—they store up food during the spring and summer and then rest for the winter. Over the spring and summer, trees use a process called photosynthesis to make food and energy. A green pigment called chlorophyll makes photosynthesis happen. During the fall and winter, there isn’t enough light or water for photosynthesis to occur, so the chlorophyll begins to fade way. As the green disappears, the other colors begin to emerge. These colors were present in the leaves all along, but they were dominated by the chlorophyll

Why does my hair stand on end when I take off my hat on a cold, dry day?

Everything you see is made up of atoms. They contain even smaller particles, called protons and electrons. Protons have positive electrical charges and they never move. Electrons have a negative charge and they move around. Atoms usually have the same number of protons and electrons, so they cancel each other out and atom is neutral—it has no charge. When two things are rubbed together, sometimes the electrons move from one thing to the other. The atom that loses electrons becomes positively charged, and the atom that gets more electrons becomes negatively charged. Two things that have different charges pull toward each other; two things with the same charges push away from each other. When you take your hat off, electrons from your hat move on to your hair. Your individual hairs then have the same positive charge, so they move away from each other, and you look really funny.

How does a plane takeoff and fly?

It’s easy to understand how a bird can fly—it’s lightweight and has wings. But how does a huge airplane get off the ground? The plane’s engine pushes the plane forward. As it moves, air flowing around the wings creates lift. The lift increases as the plane gathers speed. The plane takes off once there’s enough lift to overtake gravity. When the plane’s in the air, thrust from the engines pushes the plane forward.

How do scientists know how to make a flu vaccine if viruses can be different every year?

The flu virus changes every year. However, scientists gather information about virus mutations, or changes, before the flu-virus season begins. This lets them predict what each year’s flu virus might look like. Based on that, a vaccine is made that we hope will be accurate enough to help people fight off major cases of the flu.

What are stem cells?

Stem cells, the basic building blocks of human development, are sometimes called “magic seeds.” That’s because they can regenerate human tissue of various kinds. The use of stem cells is controversial because the best source for the cells is human embryos. Stem cells form four to five days after an egg is fertilized. These embryos must be destroyed to harvest the cells, and those opposing the research consider this the same as taking human life. Those who support stem cell research say that an embryo that is just a few days old is simply a miniscule cluster of cells and not the same as a human life. They maintain that stem cells have the potential to save human lives. Stem cells show promise in being able to one day be able to treat and cure many illnesses and diseases, such as Alzheimer's, diabetes, Parkinson's, spinal cord injuries and other medical conditions.

Why do stars twinkle?

We see the stars through the atmosphere. Their light passes through millions of miles of constantly moving pockets and streams of air, which distort the image of the stars. Even though many stars are much larger than planets, they're so far away from us that they seem smaller, like tiny dots. The distortions make it seem as if the shining lights are moving or blinking. In outer space, where there is no atmosphere, stars don't twinkle.

What causes thunder?

When a bolt of lightning shoots through the atmosphere, it heats the air to an amazing 50,000 degrees Fahrenheit in a fraction of a second. The superheated air rapidly expands, cools and then contracts, causing shock waves. These shock waves create sound waves, which we hear as thunder.

Why don't the oceans freeze?

In the Arctic and Antarctic, the oceans do freeze. The ice cap at the North Pole is entirely over ocean; the ice, however, is only a few feet deep. Oceans don't freeze solid for because they contain a lot of water, which is constantly circulating around the world. In addition, water flowing from warmer oceans (and from areas near underground volcanoes) takes off some of the chill. Another important factor is that oceans contain salt water, which has a higher freezing point than fresh water.

Why do boats float?

A steel bar dropped into water sinks, but a huge boat made of steel floats. Why? Most of the space in the boat is taken up by air. The air makes the boat less dense than water. Objects of lesser density float on liquids of greater density. This is also why holes in the bottom of a boat cause it to sink: as air floods out of the boat and water rushes in, the overall density of the boat increases to become more dense than the surrounding water.

What's the difference between bacteria and virus?

Bacteria are tiny, one-celled living organisms that can only be seen with a microscope. They live and breed in warm, moist environments in the body and elsewhere, growing quickly and causing infection. Bacterial infections can usually be treated with an antibiotic. Viruses are smaller than bacteria and cannot be seen with a microscope. They grow inside the body and produce toxins (poisons) that can cause rashes, aches and fevers. Viruses cannot be killed with antibiotics.

Why do I have brown eyes?

The genes we inherit from our parents determine things like our height, looks, hair color and eye color. This passing of characteristics from parent to child is called heredity. If your mother has brown eyes, and your father has blue eyes, there’s a good chance you’ll have brown eyes. That’s because the brown-eye gene is dominant, and the blue-eye gene is recessive. The dominant gene usually prevails over the recessive one. It’s possible, however, for you to have blue eyes if both your parents have brown eyes. They probably inherited recessive blue-eye genes from their parents and passed them on to you.