Sickle and solar panel: what they have in common

1. Step 1. Sickle — evolved into — Reaper. Mechanical reaping.
2. Step 2. Reaper — has part — Crank. A crank moves the knife.
3. Step 3. Crank — is part of — Arithmometer. The handle for adding and subtracting.
4. Step 4. Arithmometer — evolved into — Electronic calculator. Electronics replaced mechanics.
5. Step 5. Electronic calculator — has part — Solar panel. Many run on a solar cell.

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