Why Is The World's Largest Foundation Buying Fake Poop?

http://www.npr.org/blogs/health/2012/08/08/158447235/why-is-the-worlds-largest-foundation-buying-fake-poop

Graduate student Clement Cid sits atop the solar-powered toilet he helped to build at the California Institute of Technology in Pasadena, Calif. Underneath the platform, the toilet converts waste into fertilizer. The Caltech team will use fake feces to demonstrate the toilet's features next week at the Bill & Melinda Gates Foundation offices.