Hammerstone and tunnelling shield: what they have in common

1. Step 1. Hammerstone — is used in — Stone tool. A hammerstone for knapping.
2. Step 2. Stone tool — is part of — Spear. Early spearheads were made of stone.
3. Step 3. Spear — evolved into — Fire lance. The fire lance was a spear with a tube of gunpowder — the first step to firearms.
4. Step 4. Fire lance — uses — Gunpowder. A gunpowder charge.
5. Step 5. Gunpowder — evolved into — Dynamite. A replacement for gunpowder in mining.
6. Step 6. Dynamite — works like — Tunnelling shield. Two ways of driving tunnels — mechanical and explosive.

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