Security camera and railway semaphore: what they have in common

1. Step 1. Security camera — has part — Image sensor. A sensor.
2. Step 2. Image sensor — is part of — Computer mouse. The motion sensor is a tiny camera.
3. Step 3. Computer mouse — has part — LED. An optical mouse.
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.

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