The penny/nickel bar trick...

Here is a good bar 'jedi mind' trick: Give your "victim" a penny and a nickel, and ask them to put one in each hand without letting you see which coin is where. The trick is that you'll be able to tell which coin is in each hand by using the power of your mind.

Explain that they need to think about each coin so that you can read their mind. Ask your helper to multiply the value of the coin in their left hand by two without telling you the answer.

After a second's pause, ask them if they have the answer.
Then ask them to multiply the value of the coin in their right hand by 13, wait one second and ask if they have the answer.

The length of time it took them to come up with the answers to your questions will tell you which coin is in which hand. If your helper came up with both answers pretty quickly, the penny is in the left hand, (because 5 x 2 and 1 x 13 are easy to calculate). If it took a little longer to multiply by 13, then the nickel is in their right hand.

Bar Advice: Although this is a cool 'mind' trick, I don't suggest using this to pick up a boyfriend/girlfriend. Or maybe not? I just don't see this sparking a relationship... but I could be wrong. Stranger things have happened. Source here.