You ask a spectator to imagine four cards in an envelope. They make various choices - every step of the way, they are in control of what happens to the four cards - and amazingly, their chosen card is the only one reversed in the envelope! But there's more! You then reveal that the named card also has a different colored back to the other three! There's still more! You then destroy their minds completely by showing that the other three cards are actually BLANK!
The reactions are CRAZY!