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Goosebumps happen when tiny muscles at the base of each hair contract. This raises the hairs on our skin. This process, called piloerection, is controlled by the sympathetic nervous system, which also handles our "fight or flight" response. Goosebumps occur when we are cold, scared, or startled. They have two main purposes, but they don't work well for humans because we have less hair than other animals. One purpose is to keep us warm. When we are cold, the tiny muscles make our hairs stand up, creating a thin layer of air that helps insulate our bodies. This works better for animals with thick fur or feathers. For example, cold weather can make the hair or feathers of mammals and birds stand up, trapping a layer of warm air close to their bodies. The other purpose is to make animals look bigger when they feel threatened. When the muscles contract and the hair stands up, it creates a "fluffed up" look. This can make an animal seem larger and more intimidating to predators. In humans, goosebumps don't serve much purpose. We have evolved to have less hair, so goosebumps don't keep us warm or make us look bigger like they do for furry or feathered animals.