Retractable landing gear and plumbing: what they have in common

1. Step 1. Retractable landing gear — is a kind of — Landing gear. Landing gear.
2. Step 2. Landing gear — is part of — Lunar module. Legs.
3. Step 3. Lunar module — has part — Liquid-propellant rocket engine. A descent engine.
4. Step 4. Liquid-propellant rocket engine — has part — Centrifugal pump. Turbopumps deliver the propellant.
5. Step 5. Centrifugal pump — is part of — Water tower. Filled by pumps.
6. Step 6. Water tower — is part of — Plumbing. Maintaining pressure.

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