Every substance can exist in three states: as liquid, as solid state and as vapor, which mostly depend on it's temperature. This applies to water, too. So, water can be found as ice, water and steam. If water is cooled down below 0 degrees Celsius (32 Fahrenheit), it becomes ice, and if heated above 100 degrees Celsius (212 Fahrenheit), it becomes steam. The temperature, at which a substance changes it state from liquid to vapor is called a boiling point, and is different for different substances. This difference can be used to separate substances, and as such can be used for water purification.
Distillation is an effective process and, what's more important, it can be done with a lot of improvisation. You can heat water with whatever is at hand: fire, electricity, or whatever. You can use almost anything that holds water for a boiler, as long as you can direct steam into cooler, which can also be anything from a long enough pipeline to a real cooler, just that steam cools down in it. In worst case, you can distill water with an ordinary household pot and two potlids. Boil water in a pod covered with first potlid. After a while, you'll see that water in a pod vaporizes, and condenses on a potlid (this is distilled water). Just replace potlid with a second one, and turn the first one vertically, so that all condensed water collects at one point, and pour over to a cup. Meanwhile, distilled water condenses on a second potlid, so just repeat the above steps again... until you have a full cup.
Distillation will remove from water almost anything, even heavy metals, poisons, bacteria and viruses etc. However, it does not remove substances, that have boiling points at lower temperatures than water. Those are some oils, petroleum, alcohol and similar substances, which in most cases don't mix with water, and can be removed with filtering. Also, remember that substances removed from water remain in boiler, so you'll need to clean it up, every once a while.
Distilled water can be directly used, and does not need to be boiled again. As it is already hot, you can use it to prepare tea, or similar drinks. Drinking bottled distilled water alone is not recommended by official medicine as it is intended for car radiators rather than human consumption, although some alternative doctors recommend drinking clean distilled water. Try and/or decide for yourself.
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