You show your spectator several masterpieces of art printed on playing cards. One of the paintings is the famous Mona Lisa.
After showing all four paintings, you point out, that one of the paiontings is a forgery.
When you show Mona Lisa a second time, she is suddenly holding a playing card in her hand. Turning the card around, the spectators are suprised to see Mona Lisa painted from behind. She has to be the fake!
But thats not all, imagine the shock then Mona Lisa reveals she is holding the same card a spectaor has chosen!