Such is the field upon which the mouth in portraiture has been debated: an ongoing conflict between the serious and the smirk. The most famous and enduring portrait in the world functions around this very conflict. Millions of words have been devoted to the Mona Lisa and her smirk – more generously known as her ‘enigmatic smile’ — and so today it’s difficult to write about her without sensing that you’re at the back of a very long and noisy queue that stretches all the way back to 16th century Florence. But to write about the smile in portraiture without mentioning her is perverse, for the effect of the Mona Lisa has always been in its inherent ability to demand further examination. Leonardo impels us to do this using a combination of skilful sfumato (the effect of blurriness, or smokiness) and his profound understanding of human desire. It is a kind of magic: when you first glimpse her, she appears to be issuing a wanton invitation, so alive is the smile. But when you look again, and the sfumato clears in focus, she seems to have changed her mind about you. This is interactive stuff, and paradoxical: the effect of the painting only occurs in dialogue, yet she is only really there when you’re not really looking. The Mona Lisa is thus, in many ways, designed to frustrate — and frustrate she did.