A professor was giving a lecture on "Involuntary Muscular Contractions" to first year medical students. Realising that this was not the most riveting subject, the professor decided to lighten the mood slightly. He pointed to a young woman in the front row and said "Do you know what you're a-hole is doing while you're having an orgasm?" She replied "I never thought about it before, but he's probably drinking beer at the bar with his friends".