Effect

As in the classical three card monte, there's one winner card: an ace, and two other cards (e.g. two eight of hearts). 

The magician asks a spectator to try to find the ace putting the three cards face down on the table and leaving one eight apart so it's easier. Now there are only two cards to bet on, an ace and an eight. 

Spectator fails in both bets, ending with the great surprise: the eight that was left apart has now changed to an ace.

This is an unforgettable effect, since everything can be examined.