Years ago, I was taking an algebra class over the summer, and had a teacher who liked to reminisce about his past with funny stories. He relates that he had been a math teacher in the Air Force, where it was his duty to force sleepy young recruits to stay awake for an hour of math at 8:00 in the morning in a large, warm, dimly-lit auditorium. One day, he came into the auditorium and saw his class even sleepier and less attentive than usual. He realized that something drastic would have to be done.

Now this classroom was very old, and the blackboards, which had been nailed on to the walls with old black iron nails, had become loose over the years. As a result, these black nails jutted almost invisibly from the blackboard and this teacher kept banging his hands on them while erasing the board. He decided to put them to good use.