Canal and boiler: what they have in common

1. Step 1. Canal — has part — Canal lock. Locks.
2. Step 2. Canal lock — has part — Valve. Sluice gates.
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 — Boiler. A boiler.

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