Cardboard tube core and railway track: what they have in common

1. Step 1. Cardboard tube core — is part of — Toilet paper. The cardboard core of the roll.
2. Step 2. Toilet paper — is used in — Flush toilet. Hygiene.
3. Step 3. Flush toilet — uses — Sewerage. Draining the waste.
4. Step 4. Sewerage — is made of — Brick. The drains of Mohenjo-daro are lined with fired brick.
5. Step 5. Brick — is material for — Metro. The vaults of the first tunnels are brick.
6. Step 6. Metro — uses — Railway track. It runs on rails.

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