Heat transfer tubes and fitness tracker: what they have in common

1. Step 1. Heat transfer tubes — is part of — Solar water heater. They carry away heat.
2. Step 2. Solar water heater — uses — Greenhouse effect. A collector under glass.
3. Step 3. Greenhouse effect — uses — Infrared radiation. Infrared radiation is trapped.
4. Step 4. Infrared radiation — is used in — Pulse oximeter. Infrared absorption by haemoglobin.
5. Step 5. Pulse oximeter — is part of — Fitness tracker. An optical heart-rate sensor.

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