A water-clock, also known as a CLEPSYDRA, is an ancient timekeeping device that uses the gradual flow of water to measure time. It was one of the earliest timekeeping devices used by humans and was used in many cultures throughout history. The water-clock works by having a container filled with water, which is then allowed to slowly drip out of the container at a steady rate. As the water drips out, it is measured and used to keep track of time.