Here's something quoted directly from 60 Minutes video article:
TRY THIS MIND GAME FROM MARC SALEM
Tell a friend to put a penny in one hand and a nickel in the other (while not letting you see). Next tell him to multiply the value of the coin in his right hand by 13. Then have him do the same to the coin in the left. Have him add those two numbers together and tell you the sum. . . you will immediately know which hand has which coin.HOW: This mini-miracle is all based on a nonverbal cue that you pay attention to. Because it will take longer for someone to multiply 13x5 in his mind than 13x1, you simply notice which hand takes him longer to mentally multiply. . . and that'll be the nickel.