Five years ago we were in Guadalajara on family business. My wife went to buy a rosary for a co worker. She didn't have any small Mexican currency, so she offered to pay with US dollars. The nun agreed, but then she shoved the bills under a black light and promptly pronounced one of them to be fake. She raised such a ruckus, you would have thought someone was being murdered. There were six cops on the scene by the time I got there to solve the problem.
Never try to gyp a nun!
I took the bills back and paid her with Mexican currency. The cops let me keep the fake bill because they were not sure it was fake. Who in their right mind counterfeits a one dollar bill? When we got back to the house, my sons looked it over with a glass. Sure enough, it was fake. You could see wood chips in the paper.
Rufus