Toothbrush and rolling mill: what they have in common

1. Step 1. Toothbrush — has part — Handle. A plastic handle.
2. Step 2. Handle — is part of — Hammer. A handle.
3. Step 3. Hammer — evolved into — Trip hammer. A trip hammer is a hammer lifted and dropped by a water wheel.
4. Step 4. Trip hammer — has part — Water wheel. The shaft was turned by a water wheel.
5. Step 5. Water wheel — is used in — Rolling mill. The first mills were turned by water.

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