A clock that works with sunlight instead of machinery. A flat plate is marked with the hours, and a pointed piece of metal standing up from it casts a shadow; as the sun moves across the sky, the shadow swings round the plate and shows the time. People used them for thousands of years before mechanical clocks, and today you mostly see them as ornaments in parks, gardens and old university courtyards, often with a carved motto about time passing. They are countable: 'a sundial', 'two sundials'. Worth knowing why they are only ornaments now: a sundial only works when the sun is out, it needs to be built for the latitude where it stands, and the time it shows drifts away from clock time through the year — so it is honest but not exact.