Self-checkout and modem: what they have in common

1. Step 1. Self-checkout — has part — Touch layer (capacitive). A touchscreen.
2. Step 2. Touch layer (capacitive) — is part of — Smartwatch. A touchscreen.
3. Step 3. Smartwatch — has part — Pulse oximeter. A heart-rate and SpO₂ sensor.
4. Step 4. Pulse oximeter — has part — Photodiode. A photodiode.
5. Step 5. Photodiode — is used in — Fax. Scanning with a photocell.
6. Step 6. Fax — has part — Modem. It transmits an image over a telephone line.

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