You explain you're going to do the world's greatest mathematical card trick. A spectator thinks of any card in the deck. You hand her a deck, and while your back is turned the spectator transfers a few cards from the top to the bottom, as instructed by you depending on the card's value. You take the deck, look through it, and announce the thought-of card. The audience is only mildly impressed because the trick obviously involves some sort of stack or key cards and a mathematical procedure. You ask, "Do you know why this is my favorite mathematical card trick?" You then spread the cards face-up on the table and the entire deck is blank! "Because it's not mathematical. It's mindreading!"
