Solar panel and sickle: what they have in common

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

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