Effect: a hard-as-balls puzzle, that will drive everyone nuts and win you some free booze!
Start out with four quarters set up in a diamond pattern.
The mark's goal is to get the quarters in the correct pattern...WHILE FOLLOWING THESE RULES:
#1: Only slide one quarter at a time, using only one finger.
#2: no picking up, pushing, or stacking of coins (only operate within the two dimensions of the table... and no using one coin to push another)
#3: hardest of all: each time you let go of the coin you're moving, that coin must be touching at least two other coins.