Daguerreotype and fitness tracker: what they have in common

1. Step 1. Daguerreotype — is used in — Camera. The first cameras were daguerreotype cameras.
2. Step 2. Camera — has part — Exposure meter. A built-in light meter.
3. Step 3. Exposure meter — has part — Photodiode. A photocell.
4. Step 4. Photodiode — is part of — Pulse oximeter. A photodiode.
5. Step 5. Pulse oximeter — is part of — Fitness tracker. An optical heart-rate sensor.

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