Reflective metal layer and barrel: what they have in common

1. Step 1. Reflective metal layer — is part of — Wall mirror. A silver or aluminium coating.
2. Step 2. Wall mirror — has part — Window glass. Sheet glass.
3. Step 3. Window glass — is part of — Gothic cathedral. Stained glass.
4. Step 4. Gothic cathedral — has part — Vault. Rib vaults.
5. Step 5. Vault — evolved from — Arch. An extended arch.
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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