When people first looked round for some means of measuring time, they thought in terms of some material that flowed, just as time itself flowed (or so they thought). Popular options were water and sand, each of which could be persuaded to flow at a more or less steady rate. However, clocks using such materials were never very accurate – there is nothing that can flow evenly enough to measure time accurately. The breakthrough came when people stopped thinking in terms of flow and instead turned to systems with a so-called periodic or repetitive motion. Using a periodic system to measure time simply involves counting the number of oscillations, or cycles. Christiaan Huygens (1629-95) first devised a method to use a swinging pendulum to drive the hands of a clock. Pendulum clocks are not reliable enough to meet today's high standards of timekeeping. But our most advanced clocks still depend on counting oscillations to keep time.