Eraser and barrel: what they have in common

1. Step 1. Eraser — is made of — Natural rubber. Rubber.
2. Step 2. Natural rubber — is material for — IV drip. And rubber tubes.
3. Step 3. IV drip — uses — Gravity flow. The solution flows under gravity.
4. Step 4. Gravity flow — is used in — Aqueduct. Water flows down a gradient without pumps.
5. Step 5. Aqueduct — has part — Arch. Roman aqueducts rested on arcades.
6. Step 6. Arch — works like — Barrel. Staves squeezed by hoops hold their shape by the same compression as the stones of an arch.

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