When my husband and I arrived at an automobile dealership to pick
up our car, we were told that the keys had been accidentally
locked in it.  We went to the service department and found amechanic 
working feverishly to unlock the driver's side door.  As I
watched from the passenger's side, I instinctively tried the door
handle and discovered it was open. "Hey," I announced to the
technician,"it's open." The young man answered, "I already got that side."

One student was stuck and wrote: The proof of this is
left as an exercise for the reader.
The prof wrote: Did it, fell for a non sequitur and made a 
minor calculation error. You get a B+.

There's the story about the man working the night desk at the FBI office. 
They got a lot of wrong numbers, because it was similar to the pizza 
joint.
One night he answered "FBI." When the caller hesitated, he said "You meant 
to call Dominoes ..." The caller exclaimed "Wow, you guys really DO know 
everything!"