This week's Please Explains is the second in our series on how to save the world—ways to approach complex global problems such as climate change, food supply, garbage disposal, the global water supply, and violence. Today we're looking at the population explosion—there are now 7 billion people on the planet. We're joined by Hania Zlotnik, director of the population Division at the Department of Economics and Social Affairs at the United Nations, and Dr.Joel E. Cohen, mathematical biologist and the head of the Laboratory of Populations at Rockefeller University and Columbia University, and author of How Many People Can the Earth Support?