If you work retail yo most likely have a counterfeit detector pen lying around the register. This trick prays on people's insecurity about money so only do this to regular customers with a good sense of humor, I cannot be held responsible for you getting you butt handed to you. The way this works is when you mark a counterfeit bill with the pen the mark turns black otherwise it will be light brown or yellow. You have to make the mark come out black.
*Edit* This may or may not work outside the US. I'm am not certain on how to detect counterfeits elsewhere but the concept is simple and could be molded to your area most likely.
Step 1: Material Needed
Aquire a counterfeit detector pen, a black sharpie (the closer resembling to the pen the better) and choose your victim. I have done this with everything from a $1 bill to a $100 bill and people will believe it, just find someone with a good sense of humor like I said before.