There are two economists – one young and one old – walking down the street together. The young economist looks down and sees a $20 bill on the street and says,

There are two economists – one young and one old – walking down the street together. The young economist looks down and sees a $20 bill on the street and says, “Hey, look a twenty-dollar bill!” Without even looking, his older and wiser colleague replies, “Nonsense. If there had been a twenty-dollar lying on the street, someone would have already picked it up by now.”