two firemen go into a forest to put out a small fire.

Afterwards, when they emerge and go over to a stream, the face of one is all smeared with black, while the other man’s face is completely clean. 

My question is this: which of the two will wash his face?

That’s a silly question. The one with the dirty face of course.’

No, the one with the dirty face will look at the other man and assume that he looks like him. 

And, vice versa, the man with the clean face will see his colleague covered in grime and say to himself: I must be dirty too. I’d better have a wash.’