Thanks largely to an unusual molecular structure, water is amazingly good at dissolving things. (We look at this in more detailed in our main article on water.) Sometimes that's helpful: if you want to bust the dust from your jeans, simply throw them in your washing machine with some detergent and the water and soap will pull the muck away like a magnet. But there's clearly a downside to this too. All of our water constantly circulates through the environment in what's known as the water cycle. One minute it's rushing through a river or drifting high in a cloud, the next it's streaming from your faucet (tap), sitting in a glass on your table, or flushing down your toilet. How do you know the water you're about to drink—with its brilliant ability to attract and dissolve dirt—hasn't picked up all kinds of nasties on its journey through Earth and atmosphere? If you want to be sure, you can run it through a water filter.
Water filters use two different techniques to remove dirt. Physical filtration means straining water to remove larger impurities. In other words, a physical filter is a glorified sieve—maybe a piece of thin gauze or a very fine textile membrane. (If you have an electric kettle, you probably have a filter like this built into the spout to remove particles of limescale.) Another method of filtering, chemical filtration, involves passing water through an active material that removes impurities chemically as they pass through.