A flushing toilet, hooked up to a fresh water supply and a stable sewage line, is something many people in the modern world take for granted. According to the World Health Organization, approximately 2.3 billion people don't have access to the basic sanitation afforded by modern plumbing. But all those people still turn food into waste, and that waste still has to go somewhere. Unfortunately, without plumbing, it ends up going pretty much everywhere you don't want it to go, including into soil or into bodies of water, sometimes causing the rapid spread of nasty and potentially fatal diseases. This problem weighs on the minds of plenty of people, including some of the crusading heads of charitable foundations and some of the world's most inventive scientists, who have been trying to solve the problem of poop and make widespread sanitation possible. In the latter category, for example, there's Bill Gates...