Pull out a deck of cards with blue backs from a blue card case. Hold it face up and shuffle it while showing both sides of the cards. Spread the cards face up and have a volunteer choose one card. Mix that card with the rest of the deck, and spread the deck face down. All the cards' backs have changed to red, save a single exception. Show the card's face; it is the card chosen by the volunteer. You will later change the color of this card's back (and also of the card case) to red.
(Poker size- the finest Belgian-made cards/English instructions are included.)