Rabbi Yochanan, speaking in the name of Yossi the son of Zimra, asked about a verse that the eye passes over too quickly. What shall be given unto thee, or what shall be added unto thee, thou false tongue? (Psalm 120:3). The question is not rhetorical. The Holy One is actually speaking to the tongue itself.
Consider, said the Rabbi, how the tongue was made. Every other limb of the body stands upright, only the tongue lies flat inside the mouth. Every other limb is outside, exposed to sun and wind, only the tongue is tucked inward. And not content with placing it inside, the Holy One walled it behind two guards, one of bone (the teeth) and one of flesh (the lips). Twice hidden, twice fenced. What more could possibly be added to restrain it?
And yet, even with this double fortress, the tongue still escapes and wounds. So the verse becomes a kind of exasperated divine sigh. I have given you every possible obstacle, and still you slander.
Rabbi Yochanan sharpens the teaching further. The one who slanders is an atheist, he says, because such a person has declared, With our tongues will we prevail; our lips are with us; who is lord over us? (Psalm 12:4). The slanderer has thrown off the yoke of Heaven. This passage is drawn from Arakhin 15b (Harris, Hebraic Literature, 1901). The mouth is built as a prison for the tongue; when we use it to destroy a neighbor, we break out of that prison on purpose.