Barrel hoops and elevator: what they have in common

1. Step 1. Barrel hoops — is part of — Barrel. Metal hoops pull the staves together.
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 — Bridge. Stone bridges were built as arches.
4. Step 4. Bridge — includes — Suspension bridge. A bridge.
5. Step 5. Suspension bridge — has part — Wire rope. Load-bearing cables.
6. Step 6. Wire rope — is part of — Elevator. The car hangs on cables.

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