Wall heating flues and fuel cell: what they have in common

1. Step 1. Wall heating flues — is part of — Hypocaust. Pipes in the walls for hot air.
2. Step 2. Hypocaust — evolved into — Central heating. The Roman hypocaust was an early central heating system.
3. Step 3. Central heating — has part — Centrifugal pump. A circulation pump.
4. Step 4. Centrifugal pump — is part of — Liquid-propellant rocket engine. Turbopumps deliver the propellant.
5. Step 5. Liquid-propellant rocket engine — uses — Hydrogen. Hydrogen is an efficient fuel.
6. Step 6. Hydrogen — is used in — Fuel cell. The fuel is hydrogen.

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