Water purifiers is another one of those interesting and underrated categories. I remember that when I was studying anthropology, one of my professors made the claim that the single biggest historical factor impacting infant mortality rates, was access to clean water. But what does it mean for water to be clean, and how does it get that way? There are several natural resources that have valuable properties in this regard. Some substances are used as filters, meaning they capture unwanted particles as water passes through. Examples include activated carbon, charcoal, and diatomaceous earth. Some are flocculants, meaning that they adhere to undesirable particles and cause them to drop to the bottom of the water, like pebbles at the bottom of a river. Examples include alum (alunite) and aragonite (calcium carbonate). Some plants have the capacity to sequester undesirable compounds, meaning that the plant takes up the compound into its tissues and holds it there. One of the most widely used of these is the common reed (Phragmites australis).