Last Sunday I preached on the importance of our words and the nature of the heart.
Here is a helpful picture of the illustration I gave last Sunday. This illustration comes from Paul Tripp. Changing our behavior in regards to our words (or anger, or truth telling...) without getting to the heart that produces sinful words is like a man who thinks he fixed his dying, apple-less apple tree by fastening store-bought apples on it. Mat Adams made me a picture to illustrate this illustration. :)
"For no good tree bears bad fruit, nor again does a bad tree bear good fruit, for each tree is known by its own fruit. For figs are not gathered from thornbushes, nor are grapes picked from a bramble bush. The good person out of the good treasure of his heart produces good, and the evil person out of his evil treasure produces evil, for out of the abundance of the heart his mouth speaks.