It can get pretty dark under the corrugated metal roofs of Manila’s slums. Millions of families in the Philippines go without electric light, and those who have it can be at risk of fires from faulty wiring. But thanks to an innovation developed at MIT and distributed by the Liter of Light project, that problem can be solved with just a hole in the roof, a bottle of water, and some bleach. The bleach and water solution refracts sunlight, illuminating the insides of close-packed shanties in a way that windows can’t. Installation takes less than an hour and costs little, and the the light is equivalent to a 55- to 60-watt bulb.