Railway switch and rocket: what they have in common

1. Step 1. Railway switch — is part of — Railway track. Switches.
2. Step 2. Railway track — is used in — Metro. It runs on rails.
3. Step 3. Metro — uses — Tunnel. Tunnels.
4. Step 4. Tunnel — uses — Dynamite. Drilling and blasting.
5. Step 5. Dynamite — evolved from — Gunpowder. A replacement for gunpowder in mining.
6. Step 6. Gunpowder — is used in — Rocket. The first rockets were powered by gunpowder.

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