"A Math Teacher's Story" joke

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 edge of the board. Then, clearing his throat,
he drew a coathook right near one of the protruding nails. He proceeded to
hang his coat and hat on the hook that he had drawn in chalk (really
on the nail, of course). Then he went on to give that day's lecture. He told
us that the entire class had their eyes to the front of the room throughout the
lecture. He didn't know if they'd heard a single word he'd said, but at least
they looked attentive.
At the end of the class, the lecturer would usually leave by a small door near the
blackboards, while the class would leave through the large doors at the back of
the hall. When class was over, he took his coat and hat, erased the coathook,
and left through the small door-and was followed by the entire class, lining up to
go past the blackboard to see how he'd done it.
-John C. George