A deck of cards is spread face up on the table, you remove one card from the deck, and then tell a spectator to freely name any card from the deck. you then grab all the other cards turn them face down spread them out and show that there is nothing drawn on the back of any of the other cards but when you turn over your selected card they will see a drawing of a lock on it and to their amazement when their card is turned over they will see a picture of a key to unlock the lock.

whatever card they freely choose will always have a drawing and permanent marker that matches the drawing of your card. the drawing is really in permanent marker, all the cards are completely ordinary, and they're freely chosen card with the key on it is just one single ordinary card as well and the key really is drawn in permanent marker