Three Card Monte. The magician displays three cards. One is red, and the other two are blue. A spectator is asked to try and keep track of the red card after the group is flipped over and mixed. No matter how hard they try, the spectators never can find the red card - because it vanishes from the group as soon as the cards are mixed! In its place is found a joker!
This is a trick that works itself. There is no sleight of hand involved. Can be performed moments after opening the package!
Three Card Monte