Treadmill and sickle: what they have in common

1. Step 1. Treadmill — has part — Conveyor belt. A belt.
2. Step 2. Conveyor belt — uses — Belt drive. The belt runs around the drive and tension drums.
3. Step 3. Belt drive — is part of — Combine harvester. Belt drives.
4. Step 4. Combine harvester — evolved from — Sickle. The machine replaced reaping by hand.

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