What causes a ring in the bathtub?

Soap scum. When soap mixes with water, the minerals form a sticky residue that accumulates on surfaces, leaving a ring around the bathtub. Hard water—that is, water with high levels of minerals— exacerbates this problem. In addition to causing soap scum buildup, hard water also leaves stains on metals and creates a scale that covers everything from sink fixtures to showerheads.