Hypocaust and submarine: what they have in common

1. Step 1. Hypocaust — evolved into — Central heating. The Roman hypocaust was an early central heating system.
2. Step 2. Central heating — has part — Boiler. A boiler.
3. Step 3. Boiler — is part of — Steam turbine. Steam from the boiler.
4. Step 4. Steam turbine — is part of — Nuclear submarine. A turbine turns the propeller.
5. Step 5. Nuclear submarine — evolved from — Submarine. Nuclear power let submarines stay submerged for months.

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