THINK ALIKE by A.K. Dutt - Trick
The magician counts the cards and takes out the card at the selected number and places it aside. Then he instructs the volunteer to sign on it.
Magician inserts the signed card somewhere in the middle of the deck as selected by the volunteer.
When the deck is turned over and spread on the table, only one card is found with a "?" mark printed on it.
On turning it over, it was found that the volunteer has signed on the back of that card only.
Magician then tells the volunteer that it happens only if both THINK ALIKE and not otherwise.
The special deck contains 40 laminated bridge size cards. Use your own whiteboard marker.
No need of re-fills as you can repeatedly use the cards as they are laminated.