As you read the following description, consider how impossible lay people would find this effect.
The mind reader (M) gives each of two participants (P1 & P2) half a deck of cards and asks them to shuffle their packs. M turns his back and asks each P to select any ten cards. They are encouraged to take them from the top, center or bottom of their shuffled packs.
The participants are directed to hold their ten cards like an oversized poker hand and select one to hold face down against their chests. The two packets of nine remaining cards are tabled face down. M explains he wants this to be even more challenging so the two P are to slip their memorized card into the other P’s tabled 9-card pack. The Ps pick up their individual packs and mix them. Finally, P1 hands her pack to P2 who is free to place it on top of or underneath his own ten cards. M now turns around to face the Ps and takes the pack of twenty cards.
Each P is invited to hold out a hand palm up and visualize their chosen card. M takes the top card and passes it over each palm and then drops it face down on the table between P’s hands. He continues this until he pauses holding a card over one of the palms (say P2’s) then places the card face down onto the palm and says, “Maybe.”
M continues removing single cards and waving them over P1’s palm until deciding to drop one onto her palm as well. M passes his own palm over both cards and announces that something is not right. He remembers that the two cards were exchanged at the start and switches them. Finally, both P are asked to name their cards for the first time. The cards are turned over for a perfect ending.