What mysterious mark is hiding in plain sight on the Mona Lisa's protective glass? After leading more than 75 Paris tours, The Traveling Professor discovers a puzzling clue, hears a security guard's extraordinary story, and investigates.

Visiting Paris and not sure which art museums to prioritize? Start with the Louvre, Musée d’Orsay, and Musée de l’Orangerie for the city’s most memorable masterpieces.