Gravity flow and zip: what they have in common

1. Step 1. Gravity flow — is used in — IV drip. The solution flows under gravity.
2. Step 2. IV drip — has part — Syringe. A needle.
3. Step 3. Syringe — has part — Needle. A hollow needle.
4. Step 4. Needle — is used in — Trousers. Trousers were sewn with a needle.
5. Step 5. Trousers — has part — Zip. A fastener.

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