Mouth is a sawed off shotgun at any second could pop off
Words are like a gun ready to hurt any time — and they got a huge harm power which Ben’s about to release. You can get so caught up in the heat of an argument, that you might say something that you don’t mean, but it can still deeply hurt somebody.
This might be because sometimes the truth hurts, and he when he speaks the truth he doesn’t even try to sugar coat it or care about hurt feelings. Then he is able to pivot off the mouth reference to the toothpaste line which rhymes with truth taste, which goes back to the idea of his mouth being a shot gun, and he goes off on that rhyme.
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