Smoke detector and mechanical clock: what they have in common

1. Step 1. Smoke detector — has part — Photodiode. An optical scattering sensor.
2. Step 2. Photodiode — is part of — Pulse oximeter. A photodiode.
3. Step 3. Pulse oximeter — is part of — Smartwatch. A heart-rate and SpO₂ sensor.
4. Step 4. Smartwatch — evolved from — Wristwatch. The form factor of a wristwatch.
5. Step 5. Wristwatch — is a kind of — Mechanical clock. A wristwatch is a mechanical clock made small enough to wear.

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