Lighthouse lamp and rock drill: what they have in common

1. Step 1. Lighthouse lamp — is part of — Lighthouse. The lamp: oil, kerosene, then electricity.
2. Step 2. Lighthouse — has part — Rotation mechanism. It rotates the optics, setting the flash pattern.
3. Step 3. Rotation mechanism — is part of — Rock drill. It rotates the rod between blows.

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