When water, particularly hard water, dries on a glass surface, it can leave behind the impurities in the water, such as dirt and any chemicals or minerals. These show up as light colored or white spots on the surface of the glass. Water spots, at best, can disrupt the look of the glass and at worst impede clear vision through the glass. They can permanently mar the glass if the chemicals left behind are corrosive enough to etch the glass. Cleaning water spots from the glass as soon as they form, in conjunction with removing water and drying glass surfaces thoroughly after exposure to water, are the best ways to prevent water spots from forming.