Because you’re cold or frightened. When you feel chilled, the muscles around your hair follicles contract, causing the hairs to stand up to create a layer of insulation, explains Richard Potts, Ph.D., an anthropologist and the director of the Human Origins Program at the Smithsonian Institution’s National Museum of Natural History, in Washington, D.C. All mammals share this hair-raising trait. But humans don’t have enough body hair for the response to make a difference; it’s a vestigial reflex left over from when we had furry coats, Potts says. The goose bumps people get when they’re scared may be another vestigial reflex. Potts and others theorize that aeons ago, when the plentiful hair on our ancestors’ bodies stood on end, they appeared more menacing, and, he says, “predators would move on to look for less imposing prey.”