Protection of the Mouth:

An ox once entered the Chofetz Chaim's home and caused much damage. The Rebbitzen got very upset and chased the ox out of the house, and the Chofetz Chaim told her, "There is no point in getting angry. You have two options to prevent this from happening again – either always keep the door closed or, if you want to keep it open, constantly keep an eye on it." He then added, "The same is true regarding ruchnius. The yeitzer hara tries to push its way into a person's mouth in order to get him to cause damage by speaking forbidden words. One has two ways to prevent this. He can either always keep his mouth closed, or he can keep it open but constantly remain on guard to ensure he only speaks good, permitted words."