You show forty white cards with different shapes printed on them, shuffle and place them face down on the table. You ask a spectator to cut off a small packet of the deck, look at the shape he cut and remember it. Next, ask him to shuffle the packet. Then ask another spectator to take the balance of the deck from the table and place it in his pocket. All these actions are made with your back to both spectators.
Now ask the spectator to mix his packet face up on the table. Just now, you turn and explain that you will show the cards from the table, one by one, and at the exact moment he sees his shape he has to think "this is my shape." Finally you take out a note pad and draw his chosen shape.