If you have never seen the movie, Roman Holiday, with Gregory Peck and Audrey Hepburn, it is worth checking out.  Gregory Peck plays a paparazzi type American reporter in Rome who happens upon a young princess, Audrey Hepburn, who wants to be free from her real identify to experience a day in Rome.  One of my favorite scenes takes place at the Boca Della Veritae (Mouth of Truth), an ancient stone near the entrance of an old church.  Peck explains the legend to Hepburn, telling her that if a liar puts his hand inside the mouth, it would be bitten off while a truthful person would have nothing to fear.  Who wouldn’t be tentative to put their own hand in that mouth?