Sometimes, when two people try to decide who must do something, they ‘flip’ a coin.  One person says aloud which side he thinks will be facing up when the coin lands: ‘Heads’ is the side with a person’s head on it. ‘Tails’ is the other side.  As a joke one person could say ‘Heads I win, tails you lose’.  (He can’t possibly lose.  It isn’t fair.)