Tablet film coating and railway semaphore: what they have in common

1. Step 1. Tablet film coating — is part of — Tablet (medicine). A film coating.
2. Step 2. Tablet (medicine) — is used in — Aspirin. Produced as tablets.
3. Step 3. Aspirin — is made of — Coal. The raw materials of early pharmaceutical synthesis were products of coal tar distillation.
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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