You bring out a card case and an envelope with a "prize" in it.
Upon removing the cards from the case and spreading them, each one is seen to have a string attached to it. You then replace the cards in the case, leaving the strings draped outside of it, and talk about an old carnival game in which a player pulled a string and won whatever prize was on the attached card. Simulating this game, a spectator pulls one of the strings.
For the climax, the card attached to the string matches the "prize" card in the envelope making both you and your spectator a winner.
