​Why do you get goose bumps on your body, but not on your face?

Goose bumps, those pimples on the skint hat we sometimes experience when we get chilly, stressed or excited, evolved to keep us warm. Muscles in the base of each hair follicle, called arrectores pilorum, make the tiny hairs on your body stand up on end, trapping a layer of warm air next to the skin.
 
In modern humans, who have relatively little body hair, the effect is negligible, but for a shaggy caveman, it must have been like instant thermal underwear. Facial hair – even coarse beard fuzz – doesn’t have enough arrectores pilorum muscles to make the hairs stand up. That’s why your face doesn’t go pimply when the weather’s cold – though you can get goose bumps on the scalp, and, of course, on the back of the neck.