The Bocca della Verità (The Mouth of Truth) is an ancient marble mask mounted in the wall under the portico of the church Santa Maria in Cosmedin.
The mask might be an ancient sewer cover or an image of a fluvial deity. For centuries it simply leaned against the outer wall of the church, but in 1632 it was mounted under the portico.
Popular legend says that the mask will bite off the hand of liars if they stick it into the mouth of the mask.