The spongey device is made of graphite on top, with a carbon foam on the bottom. The graphite is highly porous and fractured, a crown of flakes, created by putting the material in a microwave oven and allowing bubbles to come to the surface and burst, in a way "just like popcorn," the researchers said. Graphite absorbs the sun's rays and heats up. This creates a pressure differential that sucks water from the bottom into the top, where it vaporizes. This sponge converts 85 percent of the solar energy in sunlight it absorbs into heat, the authors wrote in the study, which is extremely efficient.