There was a school where the headmaster had a particular problem. You see the little girls had all taken to wearing lipstick, which wasn’t the problem really, but what they did with it.
They would go into the bathroom and pretty themselves up with it, then, giggling, press their lips onto the mirrors and leave hundreds of these imprints on the glass. Every morning the janitor had to scrub it all off and it was becoming a real time waster.
The headmaster thinks on it then calls the little girls to assemble in the bathroom one morning. He explains how difficult it can be to clean these mirrors every day and asks the janitor to illustrate how.
On prior instructions, the janitor dips his mop into one of the toilet bowls and swipes along with the mirror with it.
Amazingly the problem with lipstick and mirrors never materialized again!