Control rod and tank: what they have in common

1. Step 1. Control rod — is part of — Nuclear reactor. The rods control the power.
2. Step 2. Nuclear reactor — is part of — Nuclear submarine. A reactor.
3. Step 3. Nuclear submarine — evolved from — Submarine. Nuclear power let submarines stay submerged for months.
4. Step 4. Submarine — has part — Periscope. A periscope.
5. Step 5. Periscope — is part of — Tank. Observation devices.

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