Two economists are walking down the street and one of them notices what appears to be a $20 bill on the sidewalk. “It’s not a real $20 bill,” the other economis

Two economists are walking down the street and one of them notices what appears to be a $20 bill 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.”