Crude, but ...

An experienced economist and a not so experienced economist are walking down the road. They see some shit lying on the asphalt.

The experienced economist: "If you eat it I'll give you $ 20,000!"
The not so experienced economist runs his optimization problem and figures out he's better off eating it so he does and collects the money.
Continuing along the same road they almost step into yet another shit.
The not so experienced economist: "Now, if you eat this shit I'll give you $ 20,000."
After evaluating the proposal, the experienced economist eats the shit and gets the money.
They go on.
Then the not so experienced economist starts thinking: "Listen, we both have the same amount of money we had before, but we both ate shit. I don't see us being better off."
The experienced economist: "Well, that's true, but you overlooked the fact that we've been just involved in $ 40,000 of trade."

From : http://netec.mcc.ac.uk/JokEc.html.