Piloerection is the physiological process where hair follicles contract, causing hair to stand upright. This reaction is often triggered by cold temperatures or emotional responses, such as fear or excitement. In humans, it is commonly referred to as "goosebumps," as the small bumps appear on the skin when the body reacts to these stimuli. This response is controlled by the autonomic nervous system and is thought to be a remnant of our evolutionary past. In animals, piloerection can make them appear larger to predators or help trap air for insulation. It is a natural reflex that serves various functions in different species.