Street light and centrifugal pump: what they have in common

1. Step 1. Street light — evolved from — Gas lamp. Gas lamps.
2. Step 2. Gas lamp — has part — Valve. A gas supply valve.
3. Step 3. Valve — is part of — Central heating. Valves on the radiators set how much hot water flows into a room.
4. Step 4. Central heating — has part — Centrifugal pump. A circulation pump.

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