Calcium and magnesium are what cause water to become “hard.” These minerals are dissolved in your home’s water from rocks beneath the earth’s surface. When the water evaporates, it leaves mineral deposits behind. Here’s a pro tip for you to avoid hard water spots: Make sure your sprinkler system is not shooting any water onto your basement windows and if you ever decide to hose down your house with tap water, make sure you squeegee that glass off!