The night before one exam, two students tied one on,
(well, actually, tied two on, one each), and managed to
sleep through the final. They realized they were in serious
trouble, so they agreed to tell the professor that they had
a flat tire on the way to the exam.
"No problem." said the Professor, "Come by my office
at 5 P.M. and I'll give you the exam then."
Feeling pretty clever, the students spent the intervening
time getting information on the exam from students who
had already taken it, and making sure they knew how
to do the problems. Coming to the professor's office
that evening, they were told, "Leave your books in
my office, and I'll put you in two separate rooms for
the exam." They were both ecstatic to see that the
Professor had given them the exact same exam taken
by the class that morning. However, there was an
additional page tacked on the end, upon which was
written, "For 50% of the grade, which tire was flat?"