Airship ballonet air bag and tank: what they have in common

1. Step 1. Airship ballonet air bag — is part of — Blimp. An internal air bag that regulates pressure and altitude.
2. Step 2. Blimp — has part — Propeller. Propellers provide thrust.
3. Step 3. Propeller — is part of — Submarine. A propeller.
4. Step 4. Submarine — has part — Periscope. A periscope.
5. Step 5. Periscope — is part of — Tank. Observation devices.

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