A water clock, also known as a clepsydra, is an ancient timekeeping device that measures time by the flow of water. It typically consists of a container that holds water, which drips at a consistent rate. As the water level decreases, markings on the container indicate the passage of time. Water clocks were used in various cultures, including ancient Egypt and Greece, before the invention of mechanical clocks. They were particularly useful in situations where other timekeeping methods, like sundials, were ineffective, such as during cloudy days or at night.