Leonardo da Vinci painted the Mona Lisa between 1503-1506, portraying Lisa Gherardini. Researchers have discovered symbols and letters in the painting's eyes which they believe were left by da Vinci to identify the subject. The letters "LV" were found in the right pupil and possibly "B" and "S" or "CE" in the left, along with the number "149" on the back. Da Vinci's sfumato technique, which blurs edges, contributes to the painting's mysterious smile that appears to change depending on the viewer's focus. Recently, an earlier copy of the Mona Lisa was discovered in the Prado Museum appearing younger than the original.