Wall calendar and optical telegraph: what they have in common

1. Step 1. Wall calendar — is made of — Paper. Calendars were printed on paper.
2. Step 2. Paper — is material for — Billboard. The poster is paper.
3. Step 3. Billboard — has part — LED. LED screens.
4. Step 4. LED — is part of — Traffic light. LEDs.
5. Step 5. Traffic light — evolved from — Railway semaphore. The first traffic light was a railway-style semaphore.
6. Step 6. Railway semaphore — evolved from — Optical telegraph. The semaphore idea.

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