Drinking water must be clean, or it can have a bad taste and may carry germs. Water from a natural source such as a stream, river or dam may be muddy or have fine organic matter floating in it. It may have germs in it. This water first needs to be filtered, then purified. Filtering takes out the tiny bits of solid matter, and purifying kills the germs. Germs are so small they cannot be seen and can get through a filtering system.
Read this guide together with the story The Mystery Woman.