Blackboards Jokes

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.
With enough of a flourish to guarantee the class's attention, he went to the
front of the room, near one more...