What causes goosebumps when you feel cold or scared?

Goosebumps are caused by tiny muscles called arrector pili attached to each hair follicle. When you're cold or frightened, your sympathetic nervous system releases adrenaline, which triggers these muscles to contract. This pulls the hair follicles upright, creating the characteristic bumpy appearance on the skin. In animals with thicker fur, this response serves two purposes: trapping air near the skin for insulation when cold, and making the animal appear larger when threatened. In humans, it's largely a vestigial reflex inherited from our hairier ancestors, since we don't have enough body hair for it to be particularly effective at either warming us or intimidating predators.