This was posted by Stan Kegel on the groaners listserv.

A physics professor at a state university in Michigan was famous for his animated lectures. He was short and thin with wild white hair and an excited expression. In lecture he would throw himself from the tops of desks and throw frisbees to students in the back row to illustrate various principles.

One day in class he was spinning on an office chair holding weights in each hand when he lost his balance and tumbled into the first row. He apologized to his class for going off on a tangent.