Two people are up on the roof of a house, cleaning a chimney. Suddenly they both fall from the chimney down in the fireplace. One of them is dirty, while the other one is clean. The clean man goes to the bathroom and cleans himself while the dirty one just gets back to work.
The dirty man sees that the other man is clean and supposes that he must be clean too, so he goes back to work. The clean man sees the dirty man and assumes that he must be dirty too, so he goes to the bathroom to clean himself.