Mechanical clock and gas lamp: what they have in common

1. Step 1. Mechanical clock — includes — Wristwatch. A wristwatch is a mechanical clock made small enough to wear.
2. Step 2. Wristwatch — evolved into — Smartwatch. The form factor of a wristwatch.
3. Step 3. Smartwatch — has part — Pulse oximeter. A heart-rate and SpO₂ sensor.
4. Step 4. Pulse oximeter — has part — LED. Red and infrared LEDs.
5. Step 5. LED — is part of — Street light. LEDs.
6. Step 6. Street light — evolved from — Gas lamp. Gas lamps.

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