She is two-and-a-half feet tall and nearly 500 years old. She hangs in The Louvre behind plate-glass - an unsigned, undated portrait of a smiling woman, the most idolised and abused woman in the history of art. She can be found in The Louvre, on the pavement in Buckingham Palace Road, on Doctor Who and on biscuit tins ... There's only one Mona Lisa but she's everywhere. Tonight Arena looks behind the Gioconda smile...