Blackboard

When the teacher walked to the blackboard one day, she noticed someone had written the word "penis" in very, very tiny letters. She turned around and scanned the class for the guilty face. Finding none, she quickly erased it and began her class.

Upon entering the classroom the next day she found printed, in slightly larger letters, the word "penis" again on the blackboard. Again, she looked around in vain for the culprit and found none, quickly erased it, and proceeded with the day's lesson.

Every morning for about a week this went on, each day's word printed increasingly larger the previous day. Finally, one day, she walked in expecting to be greeted by "the word" on the board, but instead found the words:

"The more you rub it, the bigger it gets!"