Gunpowder mill and syringe: what they have in common

1. Step 1. Gunpowder mill — has part — Water wheel. The drive.
2. Step 2. Water wheel — uses — Gravity flow. Falling or flowing water turns the wheel.
3. Step 3. Gravity flow — is used in — IV drip. The solution flows under gravity.
4. Step 4. IV drip — has part — Syringe. A needle.

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