The rules you both follow

Card values: ace is 1, number cards are their number, and jack, queen and king are all 5. Explain these values to the spectator before you start.

  1. Shuffle for real. Have the deck shuffled thoroughly. There is no setup.
  2. A secret number. The spectator silently chooses a number from 1 to 10. They do not tell you.
  3. Deal face up. Deal the cards one at a time face up into a row or overlapping line, at a steady pace. The spectator counts along silently. When they reach their number, the card that lands is their first key card.
  4. Follow the chain. The value of their key card is their next count. Starting with the card after it, they count that many cards to find their next key card, and so on.
  5. Remember the last. When the deck runs out before their next count is complete, the last key card they reached is their final card. They remember it.
  6. Count along yourself. Meanwhile, you secretly do exactly the same thing, starting with the very first card dealt as your first key card. Your chain and theirs will usually merge somewhere along the way, and from then on they are identical.
  7. The reveal. Name their final card, or push it slightly out of the line and let them confirm.

Why it works: Any two chains through the same deck have many chances to land on the same card, and once they do they never separate. The low value of picture cards keeps the chains short-stepped, which raises the odds of a merge. It is probability, not certainty.

The honest part

It fails roughly one time in six. Do not claim it cannot. If you miss, treat it as an experiment and do it again with a new shuffle. Iris would note that she only claims to be right most of the time, which is more than the weather forecast offers.

Practise counting

Deal through a shuffled deck on your own and follow your chain until you never lose count. Then rehearse while chatting. The steadier your dealing, the easier it is for the spectator to count along.

Credit where due: Devised by mathematical physicist Martin Kruskal and popularized by Martin Gardner. Our simulation of 200,000 shuffled decks, with the performer starting at the first card and the spectator choosing 1–10, found the performer landed on the spectator's final card about 85% of the time.

Next: The coin from behind the ear