Imagine! Having a spectator pick a card from a Blue deck, sign it and return it. Nothing new, right? Just wait! You then find their card but there's no signature on it. Telling them, "I can fix it," you wipe the signature back onto the face of the card, followed by letting them in on the fact that you cheated. You now show the back of the signed card. In plain view at your fingertips, the back of the signed card changes from Blue to Red. You now perform a ribbon spread showing that all the cards in the deck really are only one color (Blue). That is, all but their signed card.