Charcoal has microscopic pores that can filter out many impurities from dirty water. Increase the surface area of the charcoal to expose the pores and rinse until the water runs clear. Use a top layer of sand, soil, or cloth to filter out large material, then a thick layer (or layers) of activated charcoal that the water can drip through, soaking up impurities. The cleaner water can then be boiled to sterilize it further. This may not remove all impurities from water, but it uses the same principle as many modern filters.