Counterfeit Money

A big-city counterfeiter decided the best place to pass off his phony $18 bills would be in some small, rural, out-of-the-way town. So he got into his new, fine car, and off he went.
He found a tiny town with a single store. He entered the store and handed one of the bogus bills to the man behind the counter. “Can you change this for me, please?” he said.
The store clerk looked at the $18 bill for a short time, then smiled and told the man, “Sure, Mister. You want two $9s or three $6s?”