Check out Brian Brushwood and Scam School's Penny Prediction scam.
No matter what the setup, the very best scams are the ones that let the mark think they're on top, only to get punked in the end. That's what makes the penny prediction so great!
THE SETUP: reach into your pocket and pull out a bunch of change. Announce that you're going to make a prediction using a few pennies. After a bit of shuffling, close your fist around your prediction, and ask your mark to think of any two-digit number from 1 to 100. Once they're ready, have them perform the following simple calculations WITHOUT showing you any of their work:
* start with their two-digit number
* add up the two digits of their number, and subtract that amount from their number
* add up the digits that make up this new number
* divide that sum by two
To clarify, imagine they had picked the number 62. Here's what they'd do:
* start with their two-digit number: (62)
* add up the two digits of their number (6+2=8), and subtract that amount from their number (62-8=54)
* add up the digits that make up this new number (5+4=9)
* divide that sum by two (9/2=4.5)
Now imagine your mark's surprise when you bet him a beer that whatever final number they came up with will match the prediction pennies in your hand. They'll be convinced that your trick didn't work, and that they've got themselves a free beer coming. I mean, seriously: there's no way you can have 4.5 pennies in your fist, right?
Then imagine their shock when you open your fist and show that you're holding four and a HALF pennies. Zing!
THE WORK: The math takes care of itself. As long as you follow the above directions, the end result number will always be 4.5. To make the half penny, use some tin snips, garden shears, or a good pair of scissors. My suggestion is to make a few half pennies and keep 'em in your pocket for just the right moment.