"Each year the bucket was lined up meticulously to catch the ball -- and each year students secretly moved the bucket to one side, so that the ball crashed thunderously to the floor. Tiring of this rather stale joke, the professor traced a chalk line around the bucket. The students moved the bucket as usual, traced a chalk mark around the new position, rubbed it out and replaced the bucket in its original spot.
"'Aha!' the professor explained, seeing the faint outline of the erased chalk mark. He moved the bucket over it and released the ball -- which thundered to the floor as usual."