Gunpowder mill and rock drill: what they have in common

1. Step 1. Gunpowder mill — uses — Gunpowder. Production.
2. Step 2. Gunpowder — evolved into — Dynamite. A replacement for gunpowder in mining.
3. Step 3. Dynamite — is used in — Tunnel. Drilling and blasting.
4. Step 4. Tunnel — uses — Rock drill. Driving the Mont Cenis tunnel.

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