Friction and sickle: what they have in common

1. Step 1. Friction — is used in — Quern. Grain is crushed and ground by the friction of the stones.
2. Step 2. Quern — evolved into — Threshing machine. Mechanizing the processing of grain.
3. Step 3. Threshing machine — evolved into — Combine harvester. Combining machines.
4. Step 4. Combine harvester — evolved from — Sickle. The machine replaced reaping by hand.

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