Any puddle of water can start the process sunlight initiates the process and photosynthesis soon creates a green layer within the puddle.

And that prompted people to make maps like this, with all kinds of wonderful detail on the land, but when you get to the waters edge, the ocean looks like one giant puddle of blue paint.

Then I found out, post exposure, that this plastic bag if I put it into a filthy puddle or a stream filled with coliforms and all sorts of disgusting stuff, that that filthy water will migrate through the wall of the bag by osmosis and end up inside it as pure, potable drinking water.