Put all the red cards in one heap, and all the black cards in the other.
One of these packs you conceal in your pocket. You let any person draw a
card from the other pack, and while he is examining the card, substitute
the pack in your pocket for the one you hold in your hand. Let him place
his card in the pack you have taken from your pocket, and shuffle as
much as he pleases. On receiving back the pack, you will at once
recognize the card he has drawn by the difference of colour.