Some small-time crooks decided that people were so stupid that they would accept 18 dollar bills if somebody gave them any.
So they carefully made some plates and printed some up, and went to a small town to try them out.
They got up to a shopkeeper and talked for a while, then casually said,
"Say, can you give me change for an 18 dollar bill?"
"Sure", said the old shopkeeper. "What would you like, three 6's or two 9's?"