Motorcycle and knife: what they have in common

1. Step 1. Motorcycle — has part — Roller chain. Many motorcycles drive the rear wheel with a roller chain.
2. Step 2. Roller chain — is part of — Combine harvester. Chain drives power the working parts.
3. Step 3. Combine harvester — evolved from — Sickle. The machine replaced reaping by hand.
4. Step 4. Sickle — evolved from — Stone tool. The first sickles had flint inserts.
5. Step 5. Stone tool — evolved into — Knife. The first knives were stone flakes.

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