Final answer:
Goosebumps occur when your skin is cold due to the pilomotor reflex. When you are cold, the arrector pili muscles contract, causing the hairs on your skin to stand up straight. This helps to insulate your body and keep you warm.
Step-by-step explanation:
Goosebumps occur when your skin is cold due to the pilomotor reflex. This reflex is controlled by tiny muscles called arrector pili muscles that are attached to hair follicles in your skin. When you are cold, the arrector pili muscles contract, causing the hairs on your skin to stand up straight. This creates a layer of trapped air, which helps to insulate your body and keep you warm.
Learn more about Goosebumps and the pilomotor reflex