Firework stars and railway track: what they have in common

1. Step 1. Firework stars — is part of — Fireworks. Pellets with metal salts that give the colour.
2. Step 2. Fireworks — uses — Gunpowder. Gunpowder.
3. Step 3. Gunpowder — evolved into — Dynamite. A replacement for gunpowder in mining.
4. Step 4. Dynamite — is used in — Tunnel. Drilling and blasting.
5. Step 5. Tunnel — is used in — Metro. Tunnels.
6. Step 6. Metro — uses — Railway track. It runs on rails.

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