The smile is mysterious because it is elusive. At first you think she is smiling, but when your gaze is more fixed, the smile disappears.
Neurologist Margaret Livingstone of Harvard Medical School did a frequency analysis of Mona Lisa. She decomposed it into low- and high-frequency components. It is revealled that the lower the frequency, the clearer the smile. Thus everything's clear.
(From left to right: lowest, low, and high frequency)
This could be a really interesting project in an introductory numerical analysis or image processing course. I should try it some time myself.
btw, this also reminds me of things like 面相, or phrases like 皮笑肉不笑. So these are just composition of opposite facial expressions in different frequencies.
