The spectator is asked to choose between a red deck and a blue deck of cards. Let's pretend they chose the blue deck. The blue deck is placed inside their pocket (or handed to a friend to hold) until later.
The red deck is slowly spread between your fingers as the spectator is asked to merely think of any card they see. Keep in mind this is a 100% completely free choice!
Without ANY funny moves the card completely vanishes!
Next you ask them to remove the blue deck from their pocket, they are surprised to see that inside is one red card, the mentally chosen card. Strong enough for any Professional act, Easy enough for a Beginner.