Revolution counter and stone tool: what they have in common

1. Step 1. Revolution counter — is part of — Anemometer. It converts revolutions into speed.
2. Step 2. Anemometer — uses — Wind power. It is turned by the wind.
3. Step 3. Wind power — is used in — Windmill. The prime mover is the wind.
4. Step 4. Windmill — has part — Millstone. A windmill turns the millstones.
5. Step 5. Millstone — is part of — Quern. Grain is ground between millstones.
6. Step 6. Quern — is a kind of — Stone tool. A quern is two stones: the upper one is turned on the lower to grind grain.

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