Ok, this is an old one:
3 friends hire a room at a hotel, wish they each have to pay 10$ for. So that the total price for the room will be 30$.
But then the owner of the hotel discovers that their room just costs 25$.
So he orders the piccolo to go up to their room to give the 5$ back.
But this is a kind of clever piccolo, who understands that it will not be possible to split 5$ on 3 persons. So he decides to give the 3 friends a dollar each and then keep the rest, 2$ for him self.
Ok, that's fine, but if we think a little about this we'll find a verry strange thing.
The 3 friends have payed 9$ each, and the piccolo took 2$, but there was 30$ in the beginning. So what happened to that missing dollar?