Rare-earth metals and railway semaphore: what they have in common

1. Step 1. Rare-earth metals — is material for — Smartphone. The magnets of the speakers and vibration motor are rare-earth.
2. Step 2. Smartphone — has part — LED. The camera flash.
3. Step 3. LED — is part of — Traffic light. LEDs.
4. Step 4. Traffic light — evolved from — Railway semaphore. The first traffic light was a railway-style semaphore.

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