involuntary muscle contraction

Professor Higgins at the University of Sydney was giving a lecture on 'Involuntary Muscle Contraction' to his first year medical students.

Realising 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 behind (word swapped) is doing while you're having an orgasm?'

She replied, 'Probably out sailing with his mates!'

It took 45 minutes to restore order in the classroom.