"Two economists are walking down the street and one of them notices what appears to be a $20 bill (or a $100 bill—the monetary amounts vary) on the sidewalk. “It’s not a real $20 bill,” the other economist declares. “If it were a real $20 bill, someone would have picked it up off the sidewalk already.”
[1]: https://www.barrypopik.com/index.php/new_york_city/entry/if_...