"This is Mohammed Bah Abba's pot-in-pot invention. In northern Nigeria over 90% of villages have no electricity. His invention, which won a Rolex Award (and $100,000), is a refrigerator that runs without electricity. Here's how it works. You take a smaller pot and put it inside a larger pot. Fill the space in between them with wet sand, and cover the top with a wet cloth. When the water evaporates, it pulls the heat out with it, making the inside cold. It's a natural, cheap, easy-to-make refrigerator. So, instead of perishable foods rotting after only three days, they can last up to three weeks. Obviously, this has the potential to change lives. And it already has -- there are more girls attending school, for example, as their families no longer need them to sell food in the market."This device is based on the principles of evaporative cooling - similar to the way English butlers used to quickly cool a bottle of champagne by wrapping it with a wet towel and holding it out the window while driving the Bentley down the road. Efficacy does require a low ambient humidity, but, like the merry-go-round water pump I blogged earlier, it clearly is a useful contrivance.