TNT and electric motor: what they have in common

1. Step 1. TNT — works like — Dynamite. Two industrial explosives; TNT is safer to handle.
2. Step 2. Dynamite — works like — Tunnelling shield. Two ways of driving tunnels — mechanical and explosive.
3. Step 3. Tunnelling shield — evolved into — Tunnel boring machine. A development of the tunnelling shield.
4. Step 4. Tunnel boring machine — has part — Electric motor. Electric motors turn the cutting heads.

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