Primitive methods of telling time included observing the sun and stars, inventing sundials, and using flowing water. Sundials were among the earliest devices and worked by casting a shadow. Later, cultures developed water clocks where water flowed between containers and candle or incense clocks that burned down at a consistent rate. The development of mechanical clocks introduced pendulum technology for improved accuracy. Modern innovations include quartz clocks, atomic clocks, and digital timekeeping.