Retractable landing gear and voltaic pile: 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 — is used in — Saturn V. It launched the lunar module.
4. Step 4. Saturn V — uses — Hydrogen. Hydrogen stages.
5. Step 5. Hydrogen — uses — Electrolysis. Hydrogen is produced by electrolysis of water.
6. Step 6. Electrolysis — evolved from — Voltaic pile. The first electrolysis of water was powered by a voltaic pile.

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