Leonardo da Vinci perfected a painting technique called 'sfumato,' derived from the Italian word for 'smoke.' This method involves the incredibly 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 technique is what gives the 'Mona Lisa' her enigmatic expression; because the corners of her mouth and eyes are rendered in soft shadows, the viewer’s brain cannot pinpoint her exact emotion, making it seem to change depending on where one looks. Leonardo’s mastery of sfumato was the result of his scientific study of how light hits curved surfaces and how the human eye perceives depth, marking the perfect union of science and art during the High Renaissance.