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Explain to the audience that you will teach them a magic trick.
Have an prediction written down. Have a card freely selected by an audience member and instruct them not to look at it or show anyone.
Next you show everyone that the deck is all the same card. Lets say it is the 5 of Clubs.
Now show the prediction and it is of a different card. Let's say it is the 9 of hearts. Have the spectator look at the card for the first time and it matches the prediction.
This is the perfect sucker effect and it is entirely self working.