Christmas tree valve assembly and cannon: what they have in common

1. Step 1. Christmas tree valve assembly — is part of — Oil well. Valves at the wellhead.
2. Step 2. Oil well — is made of — Wrought iron. The casing pipes are iron.
3. Step 3. Wrought iron — is material for — Ironclad. The armour is wrought iron.
4. Step 4. Ironclad — has part — Cannon. Artillery.

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