I'm stunned by the beautiful simplicity of this wonderful idea.

Cut a hole in a developing world roof big enough for a plastic soda bottle to fit snugly into. Fill a bottle with water and a little bleach and set it halfway into the hole cap side up.

Voila, you now have a daytime light bulb. The sunshine is diffused by the water in the bottle and kicks off the light equivalent to a 50 watt bulb that costs nothing to run.

It's terrifically designed- it uses simple, abundant materials found everywhere that cost little to no money, and it can be assembled by anyone with a few simple tools and a few minutes of training. And it makes people's lives better. Perfect.

Here's a video showing how it works.

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