Millstone and solar panel: what they have in common

1. Step 1. Millstone — is part of — Windmill. A windmill turns the millstones.
2. Step 2. Windmill — evolved into — Wind turbine. A windmill with a generator instead of millstones.
3. Step 3. Wind turbine — has part — Electric generator. A generator.
4. Step 4. Electric generator — includes — Solar panel. A solar panel also generates electricity — from light rather than rotation.

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