INTERCHANGE

A magician visibly swaps four cards with a spectator. After the swap, the spectator discovers that they have exchanged the back sides of the cards, but the card values remain unchanged.

Eight cards are revealed: 4 red cards with a red back and 4 black cards with a blue back. The spectator receives 4 black cards. Both sides of the cards are shown. Now, the spectator is invited to swap the cards themselves. Due to the color difference on the back side of the cards, spectators can accurately follow the exchange. Cards are swapped one by one, always showing the back side and placing them in the opponent's deck. Then, the card is flipped to the bottom of the deck. Once all cards have been exchanged, both decks are inspected. The spectator will be amazed to find that only the colors on the backs have changed, while the values of the cards remain the same!

The magician now holds black cards with a red back and the spectator has red cards with a blue back!!!