Wheelset and refrigerator: what they have in common

1. Step 1. Wheelset — is part of — Bogie. The bogie carries the wheelsets.
2. Step 2. Bogie — is part of — Refrigerator car. Bogies.
3. Step 3. Refrigerator car — uses — Ice house. At first — ice.
4. Step 4. Ice house — evolved into — Refrigerator. The refrigerator replaced natural ice.

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