Barrel heads and syringe: what they have in common

1. Step 1. Barrel heads — is part of — Barrel. End caps.
2. Step 2. Barrel — works like — Arch. Staves squeezed by hoops hold their shape by the same compression as the stones of an arch.
3. Step 3. Arch — is part of — Aqueduct. Roman aqueducts rested on arcades.
4. Step 4. Aqueduct — uses — Gravity flow. Water flows down a gradient without pumps.
5. Step 5. Gravity flow — is used in — IV drip. The solution flows under gravity.
6. Step 6. IV drip — has part — Syringe. A needle.

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