The magician has a card selected by a spectator. The spectator is told to hold on to the card in his pocket. The magician says he has a prediction card which he produces and places face down on the table. The magician explains that his prediction card is exactly half the value of his chosen card. For example if the card he selected is the 10 of clubs the prediction card would be the 5 of clubs. The spectator is told to turn over his selected card which happens to be the 7 of clubs.The spectator now has that triumphant look on his face. The magician then asks the spectator,
"What's half of 7?"
"3 and a half."
The magician turns over the prediction card and it happens to be a 3 and a half of clubs! Amazing magic!