This is an old logic puzzle, but none the worse for it.
Two boys were happily playing on the roof of an old shed when something gave way. The two boys both fell through the roof and in a clatter of beams, chipboard and roofing felt (yes, yes, roofing felt would not clatter!) they landed in a heap on the floor of the shed.
Thankfully both were completely unhurt, although one boy had a very dirty face and the other boy's face was completely clean. They tried the door of the shed and to their relief found that it would open. They then left the shed and stood shaking in the sunlight after their adventure.
And here is the odd thing, the boy with the completely clean face ran off to wash his face. Why?
Well, the boy with the clean face saw the other boy had a very dirty face and assumed that his was the same. Presumably the boy with the very dirty face saw a completely clean face on his friend and felt no need to wash.