Why do you get goosebumps when you are cold or scared?

That prickle on your skin is an old animal reflex that no longer has fur to move.

Each hair on your body sits on a tiny muscle. When you feel cold or a strong emotion, those muscles contract and yank the hairs upright, raising little bumps. In furry animals this fluffs the coat to trap warm air or make the body look bigger to a threat. Humans kept the wiring but lost the thick fur, so the reflex fires for almost no payoff.

When a sad song gives you goosebumps on the bus, your body is running the same fur-fluffing program a frightened cat uses to puff up its tail.

Goosebumps are a vestigial reflex: useful gear for a furry ancestor, leftover decoration on you.

It is a clear, felt example that evolution leaves old machinery running even after the job disappears - your own skin carries a fossil reflex.

Goose-bumps = your inner cat trying to puff up fur you no longer have.

Learn the idea and practice English at the same time.