Experienced economist and not so experienced economist are walking down the road. They see a pile of shit lying on the asphalt.
Experienced economist: "If you eat that pile of shit 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 money.
Continuing along the same road they almost step into yet another pile of shit.
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 takes the money.
They continue walking.
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."
Experienced economist: "Well, that's true, but you overlooked the fact that we've been just involved in $40,000 of trade."