Electric fan and railway semaphore: what they have in common

1. Step 1. Electric fan — is part of — Mine ventilation. Fans.
2. Step 2. Mine ventilation — is used in — Coal mine. Airing the workings.
3. Step 3. Coal mine — is used in — Coal. Extraction.
4. Step 4. Coal — is material for — Coal gas. The gas was made by the dry distillation of coal.
5. Step 5. Coal gas — is used in — Traffic light. A gas lamp lit the signal.
6. Step 6. Traffic light — evolved from — Railway semaphore. The first traffic light was a railway-style semaphore.

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