A professor at the University of Mississippi was giving a lecture on 'Involuntary Muscular Contractions' to his first year medical students. Realizing 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 ass hole is doing while you're having an orgasm?' She replied, 'Probably elk hunting with his buddies.'
It took 45 minutes to restore order in the classroom...
I didn't know which was more funny to me, that a teacher would ask his student this question (of course he had to, as that was the root of the joke...) or the answer.