We're kicking off a series of Please Explains on how to save the world—ways to approach complex global problems such as climate change, food supply, garbage disposal, population control, and violence. Today's topic is how to protect the world's water supply. Upmanu Lall, Director of the Columbia Water Center, and Sandra Postel, founder of the Global Water Policy Project and National Geographic Freshwater Fellow join us to discuss the state of fresh water around the globe.