Drag and boiler: what they have in common

1. Step 1. Drag — is used in — Retractable landing gear. Retracting reduces drag.
2. Step 2. Retractable landing gear — has part — Hydraulic cylinder. Retraction cylinders.
3. Step 3. Hydraulic cylinder — is part of — Excavator. Hydraulic cylinders move the boom and the bucket.
4. Step 4. Excavator — has part — Boiler. The first excavators were steam-powered.

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