While funny money is a real issue, I always have to smile when I see the clerk whip out the pen and test the bill. All they are really doing is finding out if the money is printed on ordinary paper vs the material they use for real money. If you are willing to endure some pain, try this:
Take a few twentys and iron them with some spray starch.
Hand to clerk and allow to test with pen. It will test positive.
Watch reaction of clerk doing the test.
Read here to see why the test is almost worthless:
Midwest Fraud Prevention - Counterfeit Detection Units