Cellphones are everywhere in the developing world, as this Nairobi street scene shows. Bill and Melinda Gates believe the phones can be used for everything from farmer education to instant banking. Simon Maina/AFP/Getty Images hide caption

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Astrophysicist Szabolcs Marka has a radical idea: Use light as a barrier between mosquitoes and humans. He was awarded a $100,000 Grand Challenge grant by the Bill and Melinda Gates Foundation to see if the idea really works. Courtesy of the Gates Foundation hide caption

itoggle caption Courtesy of the Gates Foundation

"I want to grow up and become a police. But I need to study in a good school for that. I want to become a police to protect the country." - Fiza, 13 (India) Courtesy of Nike Foundation hide caption

itoggle caption Courtesy of Nike Foundation

Bill Gates, co-founder of the the Bill & Melinda Gates Foundation, checks out a toilet demo at the Reinvent the Toilet Fair in Seattle, Wash. The festival featured prototypes of high-tech toilets developed by researchers around the world. Bill & Melinda Gates Foundation hide caption

itoggle caption Bill & Melinda Gates Foundation

An Indian boy receives a polio vaccination from an Indian health worker in Amritsar last year. Narinder Nanu/AFP/Getty Images hide caption

itoggle caption Narinder Nanu/AFP/Getty Images