Low-Tech Solar "Liter of Light"

So amazing and simple!  A used plastic bottle filled with water and bleach or chlorine (to prevent algae from forming)  can be imbedded into a roof to provide 55 watts of light when there is sunlight outside.  Particularly useful for urban dwellings that are dark in daytime because they have no walls or windows, they can also be used in windowless latrines, and for additional light even when there is some daylight.  This  “light bulb” idea is spreading. Kind of embarrassing that we figured out how to work with fire, tungsten, and halogen, before this. What other magical innovations are right under our noses, soon to be discovered by dreamers and people of hope?