You sure know about the whole mystery that's hidden in this painting... Well there's some people who believe the Mona Lisa smiles because she's actually HORNY. When Marcel Duchamp added the moustache to this painting he wrote the letters L.H.O.O.Q. and if you read these letters in french it says "Elle as chaud au cul" which can traslate into "She has a warm ass" = "She's Horny" :)