I may have blogged about this before; I can't remember. In any event, after coming back from staying with his grandparents one time, our 3-year-old starting calling every piece of paper "five dollars." E.g. just today, he was pointing to a sheet of paper on which I was taking notes as "five dollars."
I suspect his grandparents let him read Rothbard's critique of the Federal Reserve, and so Clark now ironically calls every piece of paper money.