Armoured personnel carrier and boiler: what they have in common

1. Step 1. Armoured personnel carrier — evolved into — Infantry fighting vehicle. An armed APC.
2. Step 2. Infantry fighting vehicle — has part — Continuous track. Tracks.
3. Step 3. Continuous track — is part of — Excavator. Most excavators run on tracks.
4. Step 4. Excavator — has part — Boiler. The first excavators were steam-powered.

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