Screw thread and railway track: what they have in common

1. Step 1. Screw thread — is part of — Bolt and nut. A bolt and a nut are joined by a thread.
2. Step 2. Bolt and nut — is used in — Wrench. A wrench turns nuts.
3. Step 3. Wrench — has part — Ratchet. A ratchet wrench.
4. Step 4. Ratchet — is part of — Turnstile. A ratchet.
5. Step 5. Turnstile — is part of — Metro. Turnstiles.
6. Step 6. Metro — uses — Railway track. It runs on rails.

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