[Most American cities began life around some basic means of production, like mining or manufacturing.] People come in to work at the factory, ... and the goods from that factory are sent out to other cities or across the region. But over time, smaller services start building up within the city, like little grocery stores or gas stations, that are servicing needs within the community. The internal infrastructure gets larger and larger, and over time it becomes the biggest part of the city -- the city producing goods and services for itself.

Will Wright