… is from page 55 of the 2012 revised edition of Steven Landsburg’s 1993 book, The Armchair Economist (original emphasis):
I want to create another version of the game, in which students produce consumption goods for one another. In one class, students bake brownies; in another they do each other’s laundry. Halfway through the semester, I would lower trade barriers and allow students from one class to exchange services with those of the other.
This “international” version of the game would convey two valuable lessons. One is that trade expands opportunities. The second, and more important, is that trade is beneficial not because you get to export but because you get to import. The export business is the downside of international trade. You don’t enjoy doing the other class’s laundry but you do enjoy eating their brownies.