Think Alike by A.K. Dutt

A blank deck of cards (about 40 cards) is laid out on the table and the spectator is asked to choose a number between 1 and 40. The magician counts the cards.

takes the card with the chosen number and sets it aside. He then instructs the volunteer to sign it.

The magician inserts the signed card somewhere in the middle of the pile chosen by the spectator.

When the deck is turned over and spread out on the table, only one card is found to have a "?" printed on it. mark.

Turning it over reveals that the spectator's signature is on the back of this card!

The magician then tells the spectator that this will only happen if both parties think alike, otherwise it will not happen.