YOU think of a playing card and your spectators will read YOUR mind... "I know it's hard to believe," so you write your thoughts down on paper or on the back of your business card.
Your spectator now will actually read your mind. Let's say they think YOU thought of the Queen of Spades. Let THEM turn over the piece of paper to be amazed, because right there is written... a number?
"Oops, allow me to explain. This is my very first Deck of cards, and someone told me to mark them on the back. I marked them with numbers and, believe it or not, that number actually IS the Queen of Spades."

Remove the cards from the box and spread through them. Your spectator will see all different numbers on the backs of the cards. Turn over the Queen of Spades and it actually has the same number written on the paper or business card.