shells

“I love the moment when a new metaphor makes a hard concept suddenly clear. For me, teaching economics, the search for the perfect example is like the hunt for a perfect seashell on a beach with rough waves. Most shells are flawed, but I choose a handful of the best and carry those with me. Still, I’m watching, ready to cast aside a former favorite to make room for something closer to perfection.”

I’m holding office hours on a Monday afternoon, and I’m trying to explain to one of my microeconomic theory students why the average cost curve always slopes up when the marginal cost of producing more of a good is higher than its average cost of production. My usual example for explaining the connection between marginal and average—that an additional (i.e., marginal) quiz will pull up a student’s grade average if they do well, but pull it down if they do poorly—is just not working.

Suddenly inspiration strikes.

“Does your car have one of those digital gauges that tells you your gas mileage?”

“Yes.”

“Each additional mile you drive is a marginal mile. Suppose you are coasting downhill. You’re getting great mileage for this additional distance. What’s happening to your average mileage?”

“It’s going up.”

“And what would make your gas mileage go down?”

“Getting low mileage on the marginal mile, like if you’re accelerating.”

And in that moment we’re there. She gets it! We’re both laughing with excitement and mutual relief.