A professor was giving a lecture on "Involuntary Muscular Contractions" to his first year medical students. Realizing that this was not the most riveting subject, the professor decided to try to make it a bit more relevent.
He pointed to a young woman in the front row and said, "Do you know what your arsehole is doing while you're having an orgasm?"
"Oh, I dunno," she replied, "probably fishing & drinking beer with his mates."