Grinding wheel and oil lamp: what they have in common

1. Step 1. Grinding wheel — is part of — Grinding machine. An abrasive wheel.
2. Step 2. Grinding machine — is a kind of — Machine tool. A machine tool.
3. Step 3. Machine tool — is used in — Interchangeable parts. Identical parts can only be made on precise machine tools.
4. Step 4. Interchangeable parts — works like — Movable type. Movable type is an early case of interchangeable parts: identical letters move from line to line.
5. Step 5. Movable type — is made of — Clay. Bi Sheng’s type was made of fired clay.
6. Step 6. Clay — is material for — Oil lamp. Later — clay ones.

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