Everyone knows that feeling. Whether it's from the sound of nails on a chalkboard or the feeling of a fork scraped across teeth, we feel that physical visceral revulsion. Even just the description of the sounds/feelings are enough to trigger the hairs on the backs of our necks to stand up. It's much more than annoyance. What's the name for that reaction? Does it serve an evolutionary purpose or is it just one of the joys of being human?