If you ignore it: You'll waste gallons of water (and have to pay the bill).

Easy fix: Lift off the tank's lid and take a look. If water is overflowing into the vertical tube, you'll need to lower the water level in the tank. To do so, simply bend the arm on the float downward or, in newer models, squeeze the adjustment clips and slide the float down.

If the water level isn't the problem, add a little food coloring to the tank and wait about 20 minutes. A change in color in the toilet bowl means the flapper -- a rubber diaphragm at the bottom of the tank -- is leaking and needs to be replaced. Turn off the water supply behind the toilet and pop out the flapper with your hands. Take it to a hardware store and ask for a replacement. The new one should pop back in the same way.