Neodymium magnet and railway semaphore: what they have in common

1. Step 1. Neodymium magnet — is a kind of — Permanent magnet. A kind of magnet.
2. Step 2. Permanent magnet — works like — Electromagnet. An electromagnet attracts iron like a permanent magnet, but only while current flows.
3. Step 3. Electromagnet — is part of — Relay. An electromagnet switches the contacts.
4. Step 4. Relay — is part of — Traffic light. Relay controllers.
5. Step 5. Traffic light — evolved from — Railway semaphore. The first traffic light was a railway-style semaphore.

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