Why does your hair on your skin stand up when your cold?

When you're cold, your body responds by activating tiny muscles at the base of hair follicles, causing the hair to stand up in a process known as piloerection. This reaction is a vestigial reflex inherited from our animal ancestors, where raised hair would create an insulating layer of air for warmth or make the animal appear larger to predators. Additionally, the raised hair can help trap heat and retain warmth in the body. This response is part of the autonomic nervous system's way of helping you cope with cold temperatures.