You may think that tap water looks clean but a lot of dissolved chemicals and minerals are lurking within. The amount will vary from area to area (Hard & Soft water regions). These minerals (mainly consisting of calcium and magnesium) are what leave white residues on the glass. The water purification technology de-mineralises and de-ionises tap water to remove these impurities and produce pure water. In our opinion, Pure water is by far the best product to clean windows: The nature of water means it does not like being pure and it strives to return to its impure state. To do this it actually absorbs dirt particles from the glass thus aiding the cleaning process. Because of the aggressive strength of pure water, we do not need to use cleaning detergents on the windows which is much better for the environment. Because the impurities have been removed, we do not need to squeegee the windows, we simply rinse the glass after cleaning them with the brush and pure water to leave them to dry naturally for an excellent streak-free finish.