Chalkboard Culprit

One day a teacher went into her class room and saw the word, “penis” written in small letters on the chalkboard. She erased it and went on with the day's lesson. The next day, she came in and saw the same word on the chalkboard, but a little bit bigger. She erased it and went on with her lesson.
Each of the next several days, the teacher would come in to find “penis” on the board, a little larger each time. She went in one morning, expecting to find it again, but instead the chalkboard read: “The more you rub it, the bigger it gets.”