Involuntary Muscular Contractions
A professor was giving a lecture on "Involuntary Muscular Contractions" to 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 asked, "Do you know what your asshole is doing while you're having an orgasm?"
She replied, "He's probably down at the bar with his friends."