The magician gives the spectator a metal domino that has dots that can be moved around.
The magician now asks the spectator to use all of the dots and create a domino tile.
While the spectator is creating the domino tile, the magician takes out a card case containing 4 white cards.
When the spectator has finished the magician taps the domino tile with the 4 cards. Now the magician shows that one card has turned into the same domino tile as the spectator has created! The other 3 cards are still white.
All can be immediately examined!