Tiny muscles, big bumps
Next to almost every hair on your body is a muscle so small you'd need a microscope to see it. It's called the arrector pili (uh-REK-tor PIE-lie).
Arrector pili means hair raiser. When it squeezes, it pulls its hair up to stand straight.
As the hair stands up, the skin around it bunches into a little bump. Lots of bumps at once make goosebumps!
Most people can't make these muscles squeeze on purpose. They work on their own, like your heartbeat.
Cold and big feelings
Goosebumps often pop up when you're cold, like when you step out of a warm bath.
They can also come with strong feelings, like a scary story, a big surprise or an exciting moment. Your body sends out a rush of a chemical called adrenaline (uh-DREN-uh-lin), and it tells the tiny muscles to squeeze.
Some people get goosebumps, or chills, when they hear music they really love.
Why animals puff up
For furry and feathery animals, puffing up is super useful. When a bird or a cat is cold, its feathers or fur fluff up and trap a layer of warm air, like a puffy coat.
When an animal is scared, puffing up makes it look bigger. Think of a startled cat with its fur standing on end.
A porcupine raises its quills in a similar way. Quills are special stiff hairs.
A leftover from long ago
Long, long ago, the ancestors of people had much more body hair, so puffing it up helped them stay warm too.
People have much thinner body hair now, so goosebumps don't keep us very warm. But our bodies still do it. Scientists call a leftover like this vestigial (veh-STIJ-ee-ul).
The name goosebumps comes from geese. A goose's skin is bumpy where its feathers grow, and it looks a lot like our skin with goosebumps.