Nick Rowe was looking for the role of money in the Heuristic Macro Model, which is often used to introduce students to Trade economics. The problem he discovered is that there is only a role for money if there is friction in the model, and therefore a two-household (or household-firm or firm-firm) model makes money if not superfluous, then at least a poor substitute to direct barter.
Following is a “finger exercise” for introductory economics the way (I think) it should be taught, using that heuristic model and the Whopper story that often is used at the start of Introduction to Economics.1
Telling a Whopper
The Whopper story basically has the eager student challenging talk about scarcity and optimal reource allocation by saying, “But I can go into Burger King and get a Whopper for $1 and I can keep ordering $1 Whoppers all night.” The general Professorial Counter runs “well, that wouldn’t really happen because you run out of money and/or they run out of supplies,” which is a missed opportunity.
First, relax all of the normal constraints. You have an unlimited cash availability, and the Burger King has unlimited supplies and is open 24/7. Both the workers and any other customers are nether going to be irritated by you buying a new Whopper every five or ten minutes nor start calling their friends/hitting social media to get others to see the “spectacle” (no external incentives to start or stop). Oh, and the $1 price includes any taxes.
All Hail Declining Marginal Utility
For the first few rounds, both parties will act in keeping with the premise. You get your second, third, fourth, and even fifth Whopper and have spent $5.
You also are now less hungry than when you bought the first Whopper. That first Whopper cost you $1–and you valued it at least that much, if not more (consumer surplus >=0). The second was almost as good as the first; still well worth its dollar. The third, fourth, and fifth aren’t being eaten to satisfy hunger pangs, but you enjoy them at the $1 price.
You always have the same choice: give a dollar, get a Whopper. But sooner or later,2 you will decide that keeping that next $1 in your pocket is worth more than eating another Whopper (consumer surplus<0).