In ancient times the sun was used to measure time during the day, but sundials weren't much help after dark, so peoples around the world invented clocks that used dripping water to mark the hours. In one kind of water clock, possibly invented by the Chaldeans, a vessel was filled with water that was allowed to escape through a hole. The vessel's inside was marked with graduated lines, and the time was read by measuring the level of the remaining water. The ancient Greeks called their water clocks "klepsydra" ("water thief"), which comes from "kleptein" ("to steal") and "hydōr" ("water"). English speakers stole "clepsydra" from the Greeks in the 16th century, but actual water clocks have become increasingly rare.