When our skin is bumpy, like a plucked bird, we call it goosebumps!

But why do we get them, when we haven't been plucked? Well, they're designed to keep us warm. But how do they work?

Well, each hair has a muscle controlling it beneath the surface of the skin.

Imagine that this windsurf is the skin, and that the sail is the hair.

The hair muscle works in the same was as this rope. When it's relaxed, the sail lies flat along the floor. Similarly, when the hair muscle is relaxed, the hair lies flat along the skin.

But pulling on the rope makes the sail lift up - and the hair muscle does the same thing.

When you're cold, this muscle contracts and lifts up the hair. The skin around the base of the hair is also pulled up.

And you're left with a goosebump!

But what do these upright hairs do? Well, imagine that this confetti is air.

Adding some confetti makes it crash against the sail and cover the boards.

The upright hairs do a similar thing. Air is an insulator - the hairs trap the air next to the skin, which acts a bit like a blanket. And because the air can't escape, it keeps your body warmer!

You might not think that goosebumps make you much warmer, but thousands of years ago when we were all more hairy, they certainly did. Looking at other animals helps us to see why.

Lots of different animals raise their hairs of feathers from getting cold, because it stops the body heat from escaping.

There's another time when we get goosebumps - when we're scared! That also goes back to when we were hairy...

Animals like this security dog may raise their hairs when they feel threatened. This is because the raised hairs make them look bigger. And the bigger they look, the more chance they have of scaring away whatever is threatening them.

So although we're not as hairy as our ancestors were, we still get goosebumps to this day when we're scared.

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