At the Santa Maria in Cosmedin Church along the Piazza Bocca della Veritá .

It houses the famous Bocca della Veritá or Mouth of Truth (as featured in the Audrey Hepburn movie "Roman Holiday").
This grotesque slit-mouthed marble face was thought to have been a medieval drain cover now set into the portico. Medieval tradition had it
that the jaws would snap shut on liars - a useful way of testing the faithfulness of spouses. Well, here goes nuthin' ... owwww! I can't get it off!!! :-p