I recently flew from Minneapolis to Santa Fe, stopping over in the Denver airport. While waiting for my connecting flight I noticed something about the the airport that I had not seen before. Airports, those large enough to support multiple terminals, function like a city. Each one has restaurants, retail shops, transit (the train between terminals and moving walkways), separate lanes for faster moving traffic, and nodes of activity. The airport works like an ideal community, providing a safe environment for spontaneous interaction among the inhabitants.