Watch that Door!
Doors usually have locks; either to keep someone out or lock something in. We can refer to our mouth as a door. When it is open good things can come out or even words that murder the heart. So, it is probably a good idea to have a guard in front of our door to check what is coming out before it does.
“Set a guard, O LORD, over my mouth; keep watch over the door of my lips.” Proverbs 141:3 NKJV