Elevator safety brake and nuclear reactor: what they have in common

1. Step 1. Elevator safety brake — is made of — Wrought iron. The safety catches are wrought iron.
2. Step 2. Wrought iron — is material for — Ballast tank. The tanks are iron.
3. Step 3. Ballast tank — is part of — Submarine. Ballast tanks.
4. Step 4. Submarine — evolved into — Nuclear submarine. Nuclear power let submarines stay submerged for months.
5. Step 5. Nuclear submarine — has part — Nuclear reactor. A reactor.

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