Bus and stone tool: what they have in common

1. Step 1. Bus — has part — Diesel engine. The engine is usually a diesel.
2. Step 2. Diesel engine — is part of — Combine harvester. Self-propelled combines run on diesels.
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.

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