There are few mechanical systems as mystifying as a water softener. After all, isn’t all water soft? Fair point. But water softeners were created to combat a surprisingly common problem: hard water.

According to the U.S. Geological Survey, about 85 percent of American households have hard water, including homes with city water and well water. But what exactly is hard water and, if your home has this issue, what can you do about it?

What Is Hard Water?