Steel milling rollers and gas lamp: what they have in common

1. Step 1. Steel milling rollers — is part of — Roller mill. Pairs of steel rollers crush the grain.
2. Step 2. Roller mill — has part — Electric motor. The roller drive.
3. Step 3. Electric motor — is part of — Retractable roof. Drives.
4. Step 4. Retractable roof — is part of — Stadium. A retractable roof.
5. Step 5. Stadium — has part — Street light. Floodlights.
6. Step 6. Street light — evolved from — Gas lamp. Gas lamps.

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