In the tin-shack maze of a Kenyan slum, business can't be conducted indoors, and school classes are often held outdoors.
Now a Kenyan group is hoping 2-litre bottles filled with water, a hole in the roof and sunlight will brighten the indoors.
The soda bottle-as-lightbulb was first discovered in Brazil. Today thousands of people who can't afford electricity have converted to the water bottle lightpoint.
When the bottle is hung through a hole in the roof and filled with water and bleach, it refracts sunlight.
In Kenya, a youth group in the Korogocho slums is struggling to meet demands for the water bulb after installing the first 100 for free in April.
The slum's tin shack houses - which rent for $10 a month - typically do not have windows.