Two playes sit opposite to each other. One holds a coin behind his back. Shuffle it around a bit and show both (closed) hands. The other players have to guess which hand the coin is in. If they get the right hand then the person holding the coin has to drink. Otherwise the guesser has to drink. It is allowed to have two coins behind your back. So the other person always has to drink no matter what hand he picks. If you show your closed hands, and the other person points both hands and he's right, YOU have to drink twice. When he's wrong (and you have one coin instead of two ) HE has to drink twice..
