For an absurdly simple model of how the U.S. economy works, think of it as consisting of only two halves: Cars and trucks. Cars represent the demand side of the economy: The desire of Americans to buy the goods and services they want, need or enjoy. Trucks, specifically 18-wheelers and other big rigs, represent the supply side of the economy: The factories and shipping networks that make and distribute all those goods and services that consumers buy.