Wristwatch and modem: what they have in common

1. Step 1. Wristwatch — evolved into — Smartwatch. The form factor of a wristwatch.
2. Step 2. Smartwatch — has part — Pulse oximeter. A heart-rate and SpO₂ sensor.
3. Step 3. Pulse oximeter — has part — Photodiode. A photodiode.
4. Step 4. Photodiode — is used in — Fax. Scanning with a photocell.
5. Step 5. Fax — has part — Modem. It transmits an image over a telephone line.

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