One of magic's classic gags, now available with BLUE Bicycle brand backs! Custom printed by the US Playing Card Company. Poker size playing card.
Have the spectator select the 7 of Clubs (by whatever card force you prefer) and then divide the value of the card by two and that will match your prediction. The spectator will think that is odd because the value would be the 3 1/2 of Clubs and there is no such card.
Show the prediction to be correct, it is the 3 1/2 of Clubs.
Spectators always chuckle when they see a miss-printed card like this one.
Bicycle Playing Card
Poker Size card
Knowledge of a simple card force helpful