If your toilet sounds like it is running for a second or two every once in a while you may just have a leaky tank due to a bad flapper.
To see if your toilet tank is leaking, and therefore wasting water, simply drip some food coloring into the tank (Blue, green, and red are good choices – yellow, ummm, not so much).
Wait an hour or so. If you see the coloring in your bowl then you know your tank is leaking.
A leaky tank can be fixed by getting a new flapper at your favorite hardware store (cheap) and following the directions to replace it (easy). This is one of the easiest and least expensive home fix its there is!