DS says lots of factors contribute to a city's growth and evolution and in the case of Johannesburg in South Africa, one factor was apartheid: Johannesburg was a gold rush city but was one in which only white people were allowed to exist. So post-Apartheid South Africa has to come to terms with that fact and now it's lost its downtown despite growing so much. It's a mixture of vitality and white flight and gated communities. (What does the fact that Johannesburg's downtown became a ghost town have to do with apartheid?) People were moving to the edges and apartheid accentuated this development. (What about Mexico Cityï¿½one of the problems you cite is the average commute is two and a half hours for people who are living in those outlying regions. So who is living in this sprawl zone?) RV: The people moving into Mexico City tend to be people who until recently were working on farms. And the amount of money they're getting is dwindling and so they move to the city to improve their lives and many come to Mexico City and act as street vendors. Our image as editors of this book came from an aerial view of Mexico City. (The third of the six cities you focus on is Berlin. Half of the population there lives alone and 50% of the buildings lived in were destroyed in World War II. Now we talked about cities that are growing, but Berlin is shrinking.) DS: Berlin has had to come to terms with an aging population and has had to deal with exuberance over reunification and that exuberance hasn't happened. RV: You walk around parts of Berlin today and you see four or five story apartment blocks, and I think it has the potential to become vibrant again. You then think about cities like Detroit and other American cities which need to be redeveloped. Berlin has a degree of that. (If you read between the lines, you say part of the solution in Berlin is hipness.) DS: I think the problem with cities is there are too few remedies. Once somebody has an idea somewhere about a future city, it tends to spread like wildfire, that every city will then want that idea, and there is no one answer. (What's your favorite city in the world?) RV: I would have to go for a mixed equation of a city which has the beauty of Rome, the cosmopolitanism of London. DS: I think of Washington as a city of northern charm and southern efficiency, it's hard to get away of the mental map of London. Successful cities are the ones that offer their citizens the widest degree of choice.