Retractable roof and aerosol can: what they have in common

1. Step 1. Retractable roof — is part of — Stadium. A retractable roof.
2. Step 2. Stadium — has part — Street light. Floodlights.
3. Step 3. Street light — evolved from — Gas lamp. Gas lamps.
4. Step 4. Gas lamp — has part — Valve. A gas supply valve.
5. Step 5. Valve — is part of — Aerosol can. A valve.

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