It's something we rarely have to think about in America, but in developing countries people are lucky enough to have a roof over their heads, much less indoor lighting. Houses are dark during the day, and lack of interior light is a serious problem that, as it turns out, has a dead simple solution.
Soda bottles are collected, filled with water and bleach, then sealed and fitted into ceiling panels to act as brilliant mini skylights. This project, called Isang Litrong Liwanag, or A Liter Of Light, aims to show people across the globe how they can install these bottle lights for themselves and bring some light into their lives.
-via Ology
Sorry, had to say it...
to keep algae from forming