$7 solar oven design wins $75,000 prize

The Kyoto Box is a cardboard box, with an interior that's painted black, aluminum foil-covered top panels, and covered with clear acrylic. It gets hot enough to boil water. Its inventor, Kenyan-based Jon Bohmer, won a £51,000 prize from the Forum for the Future.

The box will be produced in a factory in Nairobi. He told CNN: "This took me about a weekend, and it worked on the first try. It's mind-boggling how simple it is."

Inventor turns cardboard boxes into eco-friendly oven