Watch strap and incandescent light bulb: what they have in common

1. Step 1. Watch strap — is part of — Wristwatch. A strap.
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 — Torch (flashlight). Modern torches use LEDs.
6. Step 6. Torch (flashlight) — has part — Incandescent light bulb. A bulb (today an LED).

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