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Water Clock

All timing devices, from the water clock to the digital watch, operate because of the fundamental principle that a regular pattern or cycle operates at a constant rate.

The water clock, or clepsydra, is one of the oldest tools created to tell time, known to have been in use in 16th century BC Egypt. Some claim that it may have been used in China as early as 4000BC.


  • Calibrate a timekeeping mechanism.


  • Per pair of students:
    4 clear plastic containers (2 per student)
    a nail
    scotch tape
    permanent markers

Key Questions

  • What happens when you remove the tape?
  • Is the clock telling how much time has passed?
  • Does the water flow out of the upper container at the same rate when it's nearly empty that it did when it was full? Why?

What To Do

  1. Poke a hole in the bottom of one container.
  2. Cover the hole with tape.
  3. Place the container with the hole inside the other container.
  4. Slowly fill up the top container with water.
  5. Get your helper ready with the stopwatch.
  6. Let it flow! Remove the tape and start the stopwatch. Mark the water level in the bottom container every 30 seconds with the permanent marker.
  7. Now it is ready to use. Remove all the water from the bottom container. Put tape over the hole and fill the top container with water again.


  • How does a pendulum clock work? How is it similar/different than a water clock?
  • Could you create the water clock without a stopwatch or clock?
  • How would the ancient Egyptians calibrate their water clocks without these tools?