Gas lamp and electronic calculator: what they have in common

1. Step 1. Gas lamp — evolved into — Incandescent light bulb. Electricity displaced gas lighting.
2. Step 2. Incandescent light bulb — is part of — Torch (flashlight). A bulb (today an LED).
3. Step 3. Torch (flashlight) — has part — Dry cell. Batteries.
4. Step 4. Dry cell — evolved into — Alkaline battery. An improved zinc–manganese cell.
5. Step 5. Alkaline battery — is part of — Electronic calculator. A calculator runs on a battery, sometimes on a tiny solar panel.

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