The Stolen Exam

A student stops by the office of one of his instructors and finds that the professor has stepped out for a moment, leaving an unguarded stack of the next day's final examinations on his desk. The student quickly steals one of the exams and disappears. Before issuing the exam, however, the professor counts them and notices that one is missing. He cuts one half-inch off the bottom of every exam prior to distributing them to the class, then fails the student who turns in a test paper longer than the rest.