Stone core and diesel engine: what they have in common

1. Step 1. Stone core — is part of — Stone tool. A pebble from which flakes are struck.
2. Step 2. Stone tool — evolved into — Sickle. The first sickles had flint inserts.
3. Step 3. Sickle — evolved into — Combine harvester. The machine replaced reaping by hand.
4. Step 4. Combine harvester — has part — Diesel engine. Self-propelled combines run on diesels.

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