Computer mouse and electrical telegraph: what they have in common

1. Step 1. Computer mouse — has part — LED. An optical mouse.
2. Step 2. LED — is part of — Traffic light. LEDs.
3. Step 3. Traffic light — has part — Relay. Relay controllers.
4. Step 4. Relay — is part of — Electrical telegraph. Relays boosted the signal on long lines.

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