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To begin, you hand a spectator your diary for examination - you point out that each day in the diary has the name of a playing card entered against it. This he can verify as he studies the diary, before putting it in his pocket.
Note: it doesn't need to be playing cards - you can fill the diary with anything you desire - people's names, phone numbers, words, symbols, etc.
Next you bring out some blank business cards. You write something on one of the cards, without showing it to the audience. This card is left on the table in full view. You will never touch it again. Note: instead of writing a prediction, you could hand out a sealed envelope containing the prediction.
Taking up the remaining business cards, you openly write different numbers on both sides of each card. A spectator now names any month (free choice) and takes the card that represents his month. He might think of MAY so he takes card number 5 - this ensures that he doesn't change his mind later.
Another audience member (or you can use the one spectator throughout) now mixes the remaining cards and lays them in a row in the table. He flips over a couple of cards (totally free choice) to ensure he has a completely random set of numbers.
The upper 4 numbers are now simply added together. They might total 20 (example on left).
"OK, please look up the 20th day in May."
The card written against that date might be the 4C.
Finally, your prediction is turned over to reveal that you wrote that very card - 4C!
Everything can be left for inspection.