Goosebumps are caused by a stress hormone called adrenaline that triggers tiny skin muscles to contract.

When you get cold or scared, your body releases a hormone called adrenaline from small glands above your kidneys. This hormone acts like an emergency alarm. It instantly tells the tiny muscles at the base of your body hairs to tighten up. When these microscopic muscles squeeze, they pull your hair straight up and pinch the skin around them. This is what creates those little bumps you see on your arms.