Mona Lisa is Lisa Gherardini

Hi,

Along with all the reasons that Oron gave you, it is also because Lonardo da Vinci was such a great artist and regarded as the greatest in Italy.

Here are a few interesting facts about the Mona Lisa 

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  Da Vinci started to work at this painting in 1503
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  He spent four years on the portrait
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  It is painted on 77 x 53 cm poplar wood
• 
  
  He never sold the portrait
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  It has been suggested that the portrait of the Mona Lisa is in fact Leonardo's female version of himself
• 
  What other names does she go by?
  Monna Lisa was her original title. Monna is a contraction for Madonna,
  Mia Donna (Madam or My Lady). It became Mona Lisa, in English,
  due to a spelling error. She is La Joconde in French, La Gioconda
  in Italian, "the merry one," referring to the smile but also possibly a
  play on her name, Lisa Gherardini del Giocondo.
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• 
  
  
  It took four years, almost the same length of time it took Leonardo to paint her,
  but now Mona Lisa has a $7.5 million dollar room of her own at the Louvre in Paris,
  France. It now hangs in the Salle de Etats. It gives the millions who come to see
  the painting more room and a better view.

Leonardo managed to paint he mouth in such a way that we are intrigued by her smile that compels you to concentrate on it. But I must relay a famous anecdote about this painting.

It seems an older woman went up to a guard at the Louvre and asked to be taken to the painting. When she got there she looked at it for a few minutes and said, "I don't see why so many people have been attracted to this painting ; I see nothing wonderful about it."

"Madam," the guard replied, "This painting is not on trial, but you are." 

I think her unexplained facial expression is what made it so famous. People developed an imagination regarding the mystery. What was going through her mind what is she thinking?

The popularity of this painting lies in the mystery surrounding it.  Even your opinion of who she was is questionable.