Insulin and sewage treatment plant: what they have in common

1. Step 1. Insulin — uses — Syringe. Injections.
2. Step 2. Syringe — is part of — IV drip. A needle.
3. Step 3. IV drip — uses — Gravity flow. The solution flows under gravity.
4. Step 4. Gravity flow — is used in — Sewerage. Waste water flows down a gradient.
5. Step 5. Sewerage — has part — Sewage treatment plant. Treatment plants.

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