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A classic packet trick by Nick Trost!
You display eight blue-backed cards and one red-backed card. You show the red-backer is an Ace, the prediction card. A spectator freely selects one of the blue-backed cards, but it does not match your prediction, it is a King instead of an Ace. This is puzzling, for now the rest of the blue-backed cards are shown to be Aces. To rectify the situation, you change your prediction card, an Ace, to a King. Now your prediction card matches the spectator's selection!
Easy to do. Complete with all the cards.