Leonardo da Vinci perfected a painting technique called 'sfumato,' derived from the Italian word for 'smoke.' This technique involves the subtle blending of colors and tones so that there are no perceptible transitions between different shapes or colors. Leonardo described it as 'without lines or borders, in the manner of smoke.' This is what gives the 'Mona Lisa' her enigmatic smile and the hazy, dreamlike quality of the background. By avoiding harsh outlines, Leonardo was able to mimic the way the human eye actually perceives light and depth. Scientific analysis has shown that in some parts of the Mona Lisa, Leonardo applied up to 30 layers of glaze, each only a few microns thick (thinner than a human hair), to achieve the perfect, smoky transition of light and shadow.