Stone core and electric generator: what they have in common

1. Step 1. Stone core — is part of — Stone tool. A pebble from which flakes are struck.
2. Step 2. Stone tool — includes — Quern. A quern is two stones: the upper one is turned on the lower to grind grain.
3. Step 3. Quern — has part — Millstone. Grain is ground between millstones.
4. Step 4. Millstone — is part of — Windmill. A windmill turns the millstones.
5. Step 5. Windmill — evolved into — Wind turbine. A windmill with a generator instead of millstones.
6. Step 6. Wind turbine — has part — Electric generator. A generator.

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