Corporal punishment (spanking, whipping, caning etc.) is a frequent theme in erotic literature. I can still remember reading "The Story of O" when I was 14 and getting aroused. But what exactly causes the erotic sensation? Is it purely the pain? The fact that the subjects buttocks and genitals are exposed? Is it the domination/submission aspect? The fact that the person being spanked or whipped sometimes becomes aroused?