It was I!!! I posted it on vod donkeys years ago, saw it on hackaday.com. There was a video about it on YouTube or somewhere. If you ever lift a clear glass filled with water in a lit room (sunshine or otherwise) you will see movement of reflected light. The water is refracting the light like a magnifying glass. Bleach is there to stop the growth of algae.
Only practical if you have a very thin roof. Like the corrugated ones in shanty towns!
