Hair standing up, also known as piloerection or horripilation, is a phenomenon where the hair on a person's body rises and becomes erect. This involuntary response is often triggered by cold temperatures, fear, or excitement.
Hair standing up serves an evolutionary purpose, as it provides insulation and helps to protect the body from the cold. In animals, it can also be a sign of aggression or a defense mechanism. Historically, the study of hair standing up has been used to understand the nervous system and the body's response to stress.