Clean Man's Face

Q: One day there was a train with only two passengers,one on each side, the train went pass a cave and ONE of the man's face got dirty. However it was the man with the clean face who washed his face, why?

A: The man with the clean face saw the dirty face on the other man and assumed that his face was dirty too, but the man with the dirty face saw the clean face, so he didn't wash it.