When I was in college, each semester there were companies that would buy back your used textbooks. One semester I brought my books to the book-buyer, who said "We'll pay you $41". He grabbed a thick stack of one-dollar bills and, with the thumb of his other hand, "buzzed" off a stack of them in about 1 second. He handed them to me. Sure enough, there were 41 one-dollar bills in the stack!
It took me a year to figure out how he did it so quickly. How did he do it?
The bills were new.