A mystery watch or mystery clock, in horology, is a timepiece whose working is not easily deducible, because it seems to have no movement at all, or the hands do not seem to be connected to any movement, etc.
One example is a type of mechanical watch where the movement is transmitted to the hands through a transparent crystal toothed wheel.
The first see-through watch, known in French as ("mysterious watch"), was invented by Hugues Rime, marketed by the French firm Armand Schwob et frère, and made in Switzerland. As an item of historical/horological value, it is preserved in various museum collections, such as the British Museum, the German Clock Museum, the International Museum of Horology, the Musée d'Horlogerie of Le Locle (Switzerland), the Musée d'art et d'histoire de Neuchâtel, the U.S. National Watch and Clock Museum, and the Vienna Clock and Watch Museum.