One day when the teacher walks into the classroom, she sees someone has written the word ‘PENIS’ (in tiny letters) on the blackboard. She scans the class looking for a guilty face. Finding none, she rubs the word off and begins class.
The next day, the word ‘PENIS’ is written on the board again; this time it is written about halfway across the board. Again she looks around in vain for the culprit. Not seeing anybody who might be a suspect, she proceeds with the day’s lesson.
Every morning for about a week, she goes into the classroom and finds the same disgusting word written on the board. Each day it is larger than the previous one, and each day she scrubs it off vigorously. At the end of the second week, she walks in expecting to be greeted by the same word on the board. But instead, she finds written on the board, the words “The more you rub it, the bigger it gets.”