Plumb bob and incandescent light bulb: what they have in common

1. Step 1. Plumb bob — is made of — Stone. The bob of a plumb line is stone or copper.
2. Step 2. Stone — is material for — Oil lamp. The first lamps were stone bowls (Lascaux).
3. Step 3. Oil lamp — evolved into — Incandescent light bulb. The electric lamp replaced oil and gas lamps.

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