The class rooms were equipped with the type wall clocks that "jumped" ahead each minute, in a very noticeable fashion. As it turned out, 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.
It became almost daily practice for students to take target practice at the clock - since this particular professor was not the most punctual, and many of the students considered him severely "absent-minded." A few well aimed erasers, and lo and behold, 15 minutes passed on the clock, 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 the students "You have 1 hour on-the-clock to complete the exam."
The professor then proceeded to collect all the erasers from around the room he could find, and then gleefully took aim at the clock. When he had successfully "jumped" the clock forward 1 hour, he closed the exam and collected all the exam papers.