Round wok bottom and mechanical clock: what they have in common

1. Step 1. Round wok bottom — is part of — Wok. A spherical bottom for quick frying.
2. Step 2. Wok — is made of — Cast iron. Woks are cast iron.
3. Step 3. Cast iron — is material for — Vending machine. Early vending machines were cast iron.
4. Step 4. Vending machine — evolved into — Parking meter. A payment machine.
5. Step 5. Parking meter — has part — Mechanical clock. A clock mechanism.

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