Flail and saw: what they have in common

1. Step 1. Flail — evolved into — Threshing machine. A replacement for threshing by hand.
2. Step 2. Threshing machine — evolved into — Combine harvester. Combining machines.
3. Step 3. Combine harvester — has part — Roller chain. Chain drives power the working parts.
4. Step 4. Roller chain — is part of — Chainsaw. The cutting chain is a roller chain with teeth.
5. Step 5. Chainsaw — evolved from — Saw. A mechanized saw.

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