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 lighten the mood slightly.

He pointed to a young woman in the front row and said, "Do  you know what your asshole is doing while you're having an  orgasm?"

She replied, "He's probably golfing with his  fucking buddies"