Scythe blade and steam locomotive: what they have in common

1. Step 1. Scythe blade — is part of — Scythe. A long curved blade.
2. Step 2. Scythe — evolved from — Sickle. Mowing while standing, with a long snath.
3. Step 3. Sickle — evolved into — Reaper. Mechanical reaping.
4. Step 4. Reaper — has part — Crank. A crank moves the knife.
5. Step 5. Crank — is part of — Steam locomotive. Connecting rods and cranks carry the piston stroke to the wheels.

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