La Bocca della Verit? (English: the Mouth of Truth) is an image, carved from Pavonazzo marble, of a man-like face, located in the portico of the church of Santa Maria in Cosmedin in Rome, Italy. The sculpture is thought to be part of a first century ancient Roman fountain, or perhaps a manhole cover, portraying one of several possible pagan gods, probably Oceanus. Most Romans believe that the 'Bocca' represents the ancient god of the river Tiber.
The most famous characteristic of the Mouth, however, is its role as a lie detector. Starting from the Middle Ages, it was believed that if one told a lie with one's hand in the mouth of the sculpture, it would be bitten off. The piece was placed in the portico of the Santa Maria in Cosmedin in the 17th century. This church is also home to the supposed relics of Saint Valentine.
The Mouth of Truth is known to English-speaking audiences mostly from its appearance in the 1953 film Roman Holiday. The film also uses the Mouth of Truth as a storytelling device since both Hepburn's and Peck's characters are not initially truthful with each other.
This scene from Roman Holiday was parodied in the 2000 Japanese film Sleeping Bride by Hideo Nakata. It was also replicated in the film Only You starring Robert Downey Jr. and Marisa Tomei.Read article at Wikipedia