Next time you pour yourself a cool, clear glass of water, hold it up to the light and look very closely. Do you see any pollution? Any spilled oil? Stray bacteria? Toxic chemicals? Bug-eyed microscopic creatures? Probably not—and nor are you likely to. We assume the water we put inside ourselves is as clean as clean can be, and mostly it is. But think about things logically, and you might start having doubts. We know that water pollution exists (often in quite large quantities) and we also know that the world's water is constantly recycling itself. So how can we be sure that the water we drink is perfectly clean and healthy?