Electric motor and gas lamp: what they have in common

1. Step 1. Electric motor — is part of — Retractable roof. Drives.
2. Step 2. Retractable roof — is part of — Stadium. A retractable roof.
3. Step 3. Stadium — has part — Street light. Floodlights.
4. Step 4. Street light — evolved from — Gas lamp. Gas lamps.

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