Anyone who’s visited the slums of the world’s sprawling cities knows that even under the brightest sun of Brazil or Nigeria, the shacks are dark inside. Few people can afford electricity to power lights during the day, and makeshift skylights are almost sure to leak under tropical downpours. But a solution first developed more than a decade ago by Alfredo Moser, a mechanic in Brazil, is spreading light around the world. Basically Moser’s solar “bulb” is nothing but a common clear-plastic one-liter water bottle. It’s filled with water and a bit of chlorine to keep algae from growing inside. Then it’s embedded in a hole cut in the corrugated tin or other material used for roofing. About a third of the bottle is on the outside, catching the sun, and about two thirds is inside, spreading the equivalent of 40 to 60 watts of refracted light. A group called MyShelter in the Philippines, which has brought this illuminating device to about 140,000 families, is spreading the elementary technology throughout Asia and Africa. To paraphrase a famous saying, it is better to install one bottle bulb than to curse the darkness.