Way back when, there was a not-well-educated man that ran a small town's local pool. He was getting tired of opening on mondays. I'm not sure exactly why; maybe it was due to his health, or just out of laziness. Well, he hung a sign on his door. It said, written in barely-legible letters:
"NOW NO SWIMS ON MON".
He put a nail stright through the center of the "I".

Well, a bunch of boys wanted to swim on Monday. So when they saw this sign, they were pissed. They took it and spun it 180 degrees around the nail.
It then said:
NOW NO SWIMS ON MON

The boys, amazed, went home.