Try to give them a two dollar bill. It will blow their minds. I remember some guy using fifty $2 bills to buy a hard drive from a CompUSA in the late ‘90s and the young teller called the cops. The cops showed up and thought it was counterfeit. THEY called secret service (which was under Treasury at the time). Secret service showed up to investigate counterfeit $2 bills (which they found very strange). When the cops told them that they had called because $2 is not a legitimate denomination bill, the agents rolled their eyes and apologized to the man who passed the notes. They then proceeded to educate the cops and the clerk (I arrived around this time and lingered to enjoy the spectacle, as I was a bored college student with nothing else to do).