String vibration and heat pump: what they have in common

1. Step 1. String vibration — is used in — Harpsichord. Strings.
2. Step 2. Harpsichord — has part — Keyboard. A keyboard.
3. Step 3. Keyboard — is part of — Cash register. Sums were entered with keys.
4. Step 4. Cash register — is used in — Supermarket. Tills.
5. Step 5. Supermarket — uses — Refrigerator. Refrigerated displays.
6. Step 6. Refrigerator — works like — Heat pump. A heat pump is a refrigerator that heats the house.

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