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The most common reasons are cold and fear. “We get goosebumps because our hairs are raised by the action of tiny muscles in the skin. These muscles come into action under the command of certain parts of the brain, responsible for fear reactions”, says neurologist Benito Pereira Damas, from Unicamp. The mechanism is a legacy of primitive times: when an animal feels threatened, it erects its hair, increasing in size to appear to its opponent as a more powerful enemy than it is. The cold, on the other hand, a sensation transmitted by the nerve endings of the skin, provokes, as a reflex, the occurrence of muscular contractions throughout the body: the muscles tremble, the teeth chatter and the hairs stand on end, increasing the amount of air between them. The goal is to retain heat. A high-pitched, irritating sound, such as chalk on a blackboard, can also trigger the reflex.