ACT I / THE 21-CARD MYSTERY
Remember one card.
Keep it to yourself.
Look at the three columns below. Remember any one card, then choose its column. Keep the same card in mind through all three deals.
Deal 1 of 3 · Which column contains your card?
THE REVEAL
Was this your card?
It works when you keep the same card and select its correct column on each deal. Picked the wrong column? Start again — no mysterious forces involved.
Show me the method
Three deals. A middle pile. A fixed place.
Deal 21 cards into three columns, one card per column in turn. Ask which column holds the remembered card. Gather the columns into a packet with the chosen column between the other two, keeping each column’s top-to-bottom order. Deal and gather this way three times. The remembered card will be the eleventh card in the packet.
This page uses that same procedure. It does not secretly record your chosen card at the start; each column choice narrows its possible position. To practise on a table, lay out the columns neatly and use the same card throughout.
For the mathematics behind this familiar effect, see Dibyajyoti Deb’s paper on the 21-card trick ↗.