Hard water stains on glass are caused by the presence of dissolved minerals, primarily calcium and magnesium, in the water. When hard water evaporates on glass surfaces, these minerals are left behind, forming stubborn, white, cloudy deposits known as hard water stains. These stains can accumulate over time and are particularly noticeable on glass surfaces such as shower doors, windows, and glassware. Factors such as the mineral content of the water supply, frequency of water exposure, and the presence of soap scum can exacerbate the formation of hard water stains. Removing these stains typically requires the use of acidic or abrasive cleaners designed to dissolve or scrub away the mineral deposits without damaging the glass surface. Regular maintenance and the use of water-softening systems can also help prevent the buildup of hard water stains.