The principal had a problem with some girls who were starting to use lipstick. When applying it in the bathroom they would blot their lips on the mirrors, leaving lip prints.
Before it got out of hand, he thought of a way to stop it. One day he gathered together all the girls who wore lipstick. He then took them into the bathroom and lectured about how hard it was to clean the lipstick off the mirrors. The principle then asked the custodian, who was present, to demonstrate.
The custodian took a long handled brush, dipped it into the toilet and vigorously rubbed the lipstick off the mirror.
From that day forward, the mirrors stayed lipstick free.