Espresso machine and tank: what they have in common

1. Step 1. Espresso machine — has part — Boiler. A boiler.
2. Step 2. Boiler — is part of — Excavator. The first excavators were steam-powered.
3. Step 3. Excavator — has part — Continuous track. Most excavators run on tracks.
4. Step 4. Continuous track — is part of — Tank. Tracks let a tank cross trenches and rough ground.

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