Why does your hair stand on end when you're cold, frightened, or suddenly moved by a piece of music? Tiny muscles hidden beneath your skin are responsible.

Nearly every body hair is attached to its own microscopic muscle called an arrector pili. When your nervous system detects cold, fear, or another powerful emotional stimulus, these muscles contract, pulling the hairs upright and creating the familiar goosebumps on your skin.

In humans, the response serves little practical purpose. But in our furry ancestors, and in many animals today, raised hair traps an insulating layer of air and makes the animal appear larger to predators or rivals.

That's why a startled cat suddenly seems twice its normal size. The same tiny muscles producing your goosebumps are lifting every hair in its coat.

Goosebumps may not keep us much warmer anymore, but they are a remarkably visible reminder that our nervous system still carries reflexes inherited from our much hairier past.