Metro and turbine: what they have in common

1. Step 1. Metro — uses — Railway track. It runs on rails.
2. Step 2. Railway track — uses — Rolling mill. Rails are rolled.
3. Step 3. Rolling mill — uses — Water wheel. The first mills were turned by water.
4. Step 4. Water wheel — evolved into — Turbine. The water turbine evolved from the water wheel.

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