Bocca della Verità (Mouth of Truth)

The Bocca della Verità is, in origin, startlingly mundane: a large carved marble disc, nearly 1.75 metres across, depicting a bearded river-god face, most likely made in the 1st century AD as a functional drain cover — possibly for a nearby temple, though its exact original purpose is still debated by archaeologists. It's been mounted in the portico of the church of Santa Maria in Cosmedin since 1632, which is where its legend, rather than its function, really took shape.

Medieval Roman tradition held that the mouth would snap shut and bite off the hand of anyone who told a lie while their hand was inside it — a folk lie-detector test reportedly used to settle disputes and extract confessions, with (unverified) stories of clergy hiding behind the disc to grab and 'punish' the hands of the dishonest, a piece of medieval theatre dressed up as divine judgment.