Valve and kiln: what they have in common

1. Step 1. Valve — is part of — Central heating. Valves on the radiators set how much hot water flows into a room.
2. Step 2. Central heating — evolved from — Hypocaust. The Roman hypocaust was an early central heating system.
3. Step 3. Hypocaust — has part — Firebox. A furnace outside the building.
4. Step 4. Firebox — is part of — Kiln. A combustion chamber.

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