When Jon Bohmer sat down with his two little girls for a simple project they could work on together, he didn't realize they'd hit upon a solution to one of the world's biggest problems for just $5: A solar-powered oven. The ingeniously simple design uses two cardboard boxes, one inside the other, and an acrylic cover that lets in the sun's rays and traps them. Black paint on the inner box, and silver foil on the outer one, help concentrate the heat.