Two cards are shown face up and face down by the magican. While face down, one of the cards is put into the spectator's hand and the other card is turned face up and held by the magicain behind his back. The spectator is asked to name the card in their hand. Having seen the magician's card, it is obvious that the card held by the spectator is the other card. But when the face down card is turned over and the magician's card is returned to view, the cards have switched places!
Easy to do. Everything you need is supplied.