Effect: Surprise your friends by telling them you have X-ray vision
Need: a group of people, 5 bottle caps, 5 coins, one drink, one napkin, and a secret helper.
Method: Watch as Brian successfully determines which bottle cap is missing a coin. But in reality someone is secretly helping Brian by using a code known to both of them.
Depending where the helper places his drink on the napkin, Brian will know the correct answer!