The rules at a particular university were such that if the professor were not
present in the classroom by 15 minutes past the hour, the class was considered a
"walk" and the students were free to leave, with no penalties for missing a
class. The rooms were equipped with the type wall clocks which "jumped" ahead
each minute, in a very noticeable fashion. As it were, these clocks were also
not of the most sophisticated construction. Some enterprising student discovered
that if one were to hit the clock with chalkboard erasers, it would cause the
clock to "jump" ahead 1 minute.
So, it became almost daily practice for these students to take target practice
at the clock (as it would have it, this particular professor was not the most
punctual, and the students considered him severely "absent-minded"). A few well
aimed erasers, and lo, 15 minutes were passed, and class dismissed itself.
Well, when the day for the next exam rolled around, the professor strolled
into the room, passed out the exams, and told them, "You have 1 hour to complete
The professor then proceeded to collect the erasers from around the room,
gleefully took aim at the clock. When he had successfully "jumped" the clock
forward 1 hour, he closed the class and collected the exam papers.
Life does teach some lessons the hard way!