Portable speaker and greenhouse: what they have in common

1. Step 1. Portable speaker — uses — Bluetooth. Bluetooth.
2. Step 2. Bluetooth — is part of — Fitness tracker. A link to the phone.
3. Step 3. Fitness tracker — has part — Pulse oximeter. An optical heart-rate sensor.
4. Step 4. Pulse oximeter — uses — Infrared radiation. Infrared absorption by haemoglobin.
5. Step 5. Infrared radiation — is used in — Greenhouse effect. Infrared radiation is trapped.
6. Step 6. Greenhouse effect — is used in — Greenhouse. Glass lets light in and holds heat.

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