High school biology teachers teach basic anatomy with plastic replicas of the human today. These models have all the major organs: the heart, the liver, the kidneys, and so on. The models allow teachers to show their students in a simple way how the important parts of the body fit together. Of course, no one would mistake these plastic models for a real person. These models are stylized, and they omit many details. Yet despite this lack of realism indeed, because of this lack of realism studying these models is useful for learning how the human body works. Economists also use models to learn about the world, but instead of being made of plastic, they are most often composed of diagrams and equations. Like a biology teacher,s plastic model, economic models omit many details to allow us to see what is truly important. Just as the biology teacher’s model does not include all of the body’s muscles and capillaries, an economists model does not include every feature of the eco...