Alfredo Moser's invention is lighting up the world. In 2002, the Brazilian mechanic had a light-bulb moment and came up with a way of illuminating his house during the day without electricity - using nothing more than plastic bottles filled with water and a tiny bit of bleach.
In the last two years his innovation has spread throughout the world. It is expected to be in one million homes by early next year.
So how does it work? Simple refraction of sunlight, explains Moser, as he fills an empty two-litre plastic bottle.
"Add two capfuls of bleach to protect the water so it doesn't turn green [with algae]. The cleaner the bottle, the better," he adds.
Wrapping his face in a cloth he makes a hole in a roof tile with a drill. Then, from the bottom upwards, he pushes the bottle into the newly-made hole.
"You fix the bottle in with polyester resin. Even when it rains, the roof never leaks - not one drop."