Concrete dam wall and barrel: what they have in common

1. Step 1. Concrete dam wall — is part of — Dam. A mass of concrete that holds back the water by its weight.
2. Step 2. Dam — is made of — Roman concrete. Ancient dams; modern ones are made of concrete.
3. Step 3. Roman concrete — is material for — Amphitheatre. The Colosseum.
4. Step 4. Amphitheatre — has part — Arch. Arcades.
5. Step 5. 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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