You take a deck of cards out of your card case. Then have the player choose a card and return it face down to the deck.

The spectator now counts the cards, looks through them, and notices that his card has disappeared. - The card lying face down in the deck now bears the inscription: "Please look in the card case!"

Your helper follows this instruction and opens the case, which has been in full view on the table the whole time. And inside is --- the chosen card!

Complete, but without a deck of cards, you can use your own.