A small pay envelope with a square hole cut right through its centre is displayed and a selected card slipped inside where it is clearly shown. A second card is then chosen and cut into the deck's centre. Moments later, the deck is ribbon spread face down to reveal the first selection now face up in the deck centre. Immediately the card inside the envelope is removed to reveal it is actually now the second chosen card.
This incorporates a simple yet devious envelope principle that you will be able to use in other effects.