In fact, all mammals will have the reaction of hair standing upright when they are cold or dangerous. The difference is that human hair is too short to protect the body from harm. When naked skin is entangled with hair, small pimples will form on the surface, which looks like hairless chicken skin. That's why goose bumps get their name. In fact, human goose bumps are exactly the same as the principle that a cat's hair stands up and its tail thickens when threatened. It's just that human beings invented clothes that can keep out the cold and learned to make weapons. The cold protection function of hair is gradually reduced, and this reflection activity is gradually degraded.
When you feel cold, nervous and scared, you will have such a reaction. Of course, when you are an adult, some people will wake up at orgasm.