Chainsaw and sewage treatment plant: what they have in common

1. Step 1. Chainsaw — evolved from — Saw. A mechanized saw.
2. Step 2. Saw — is used in — Sawmill. The cutting tool is a saw.
3. Step 3. Sawmill — has part — Water wheel. Early sawmills were driven by water wheels.
4. Step 4. Water wheel — uses — Gravity flow. Falling or flowing water turns the wheel.
5. Step 5. Gravity flow — is used in — Sewerage. Waste water flows down a gradient.
6. Step 6. Sewerage — has part — Sewage treatment plant. Treatment plants.

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