You ask the spectator to THINK of any card in the deck. Don't say it yet, just think it. You tell them to imagine that random card reversed - face down - in a face up deck. They can change their mind and think of a different card. It is truly a free thought.
You pull out a deck of cards from your back pocket, the spectator names their thought of card, you open the box and spread through it.
ONE card is face down in the face up deck. The card is shown to be the card that they previously THOUGHT of. Impossible. And very, very easy to learn.