Involuntary Muscular Contractions (bit naughty)

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

Spoiler:

he's probably drinking beer at the bar with his friends".