An economist and an accountant are walking along a large puddle. They get across a frog jumping on the mud. The economist says: "If you eat the frog I'll give you $20,000!"
The accountant checks his budget and figures out he's better off eating it, so he does and collects money.

Continuing along the same puddle they almost step into yet another frog. The accountant says: "Now, if you eat this frog I'll give you $20,000."
After evaluating the proposal the economist eats the frog and gets the money.

They go on. The accountant starts thinking: "Listen, we both have the same amount of money we had before, but we both ate frogs. I don't see us being better off."
The economist: "Well, that's true, but you overlooked the fact that we've been just involved in $40,000 of trade."