Water wheel and atomic clock: what they have in common

1. Step 1. Water wheel — uses — Gravity flow. Falling or flowing water turns the wheel.
2. Step 2. Gravity flow — is used in — Water clock (clepsydra). Water flows out at a constant rate under gravity.
3. Step 3. Water clock (clepsydra) — evolved into — Mechanical clock. Mechanical clocks replaced water clocks, swapping the flow of water for an escapement.
4. Step 4. Mechanical clock — evolved into — Quartz clock. Quartz clocks replaced mechanical ones: a vibrating quartz crystal took the place of the pendulum and balance.
5. Step 5. Quartz clock — evolved into — Atomic clock. The quartz oscillator is tuned to an atomic standard.

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