As fate would have it, Miss Jones was the new substitute teacher for little Johnny’s class. On her first day, she walked into the classroom and saw the word PENIS written in the corner of the blackboard. Without saying anything, she just got an eraser and rubbed it off and continued with the class.
The next day the same thing occurred except that the word PENIS was written much larger on the board. Again, Miss Jones just ignored the situation, taking an eraser and rubbing it off again.
This went on for a couple days more with the same routine until Miss Jones came in and saw the word PENIS taking up about half the blackboard. At this point she felt she had to say something. She turned towards the class and in a loud voice said “OK, someone tell me what does this mean?
There was a prolonged silence until little Johnny finally raised  his hand. “Yes Johnny” said Miss Jones. Little Johnny paused for a moment and then said “I think it means that the more you rub it, the bigger it gets.”
Miss Jones fainted.