The world needs a better toilet—one that can work for the millions of people in developing countries who lack running water and sanitary waste-removal systems. (In India alone more than 600 million citizens don’t have access to toilets or latrines, and open defecation leads to contaminated water and chronic—sometimes deadly—health issues, like diarrhea.) So far, the prototype with the most potential to address developing world needs is a self-contained solar toilet developed by researchers at the California Institute of Technology. The team’s design uses solar energy (captured with photovoltaic panels) to power an electrochemical reactor that converts water and human waste into fertilizer and hydrogen. The hydrogen can be stored in fuel cells as energy, and the sterilized water can be reused for irrigation. The Bill & Melinda Gates Foundation awarded $100,000 to the Caltech project, which is being field-tested in India this year. The next challenge—and this may be the bigger one—is to bring down the price. The prototypes cost more than $1,000 to build, putting them well out of range for the 2.5 billion people worldwide who need them.