Secret setup
Before anyone is watching, arrange the deck so the colors alternate all the way through: red, black, red, black. Suits and values do not matter. Put it back in its box. Write your prediction: “You will create twenty-six perfect couples. Every one will be one red and one black.” Fold it and set it aside.
- Introduce the prediction. Put the folded paper on the table in plain view. Do not explain it.
- The deal. Hand the spectator the face-down deck. Ask them to deal cards one at a time into a pile on the table, stopping whenever they like, ideally around the middle.
- The riffle. Ask them to riffle shuffle the dealt pile into the rest of the deck, once. If they cannot riffle, they can push the two halves together on the table in any interleaved way.
- Pairs off the top. Take the deck and deal the top two cards face up as a pair. One red, one black. Deal the next two. One red, one black. Continue through the whole deck, or let them deal.
- Read the prediction. Ask them to open the paper and read it aloud.
Why it works: Dealing the pile reverses its order, so the top card of each half is a different color. Any single riffle shuffle of two alternating halves with opposite colors on top keeps every consecutive pair mixed. This is Gilbreath's first principle.
The one rule
Exactly one riffle shuffle. A second shuffle destroys the pattern. The deal before the shuffle is also essential; if they simply cut the deck in half and riffle, the halves might start with the same color and some pairs will match.
Present it as a story
Iris would present this as matchmaking: the deck is a dance floor, the shuffle is the chaos of the evening, and somehow everyone leaves with a partner of the opposite color. Keep the reveal slow, and let them deal the last pairs themselves.
Credit where due: The principle is named for mathematician Norman Gilbreath, who published it in 1958. We checked this handling by simulating 20,000 random deals and riffle shuffles; every one produced 26 mixed pairs.
Next: The nine-card spelling trick