Steel shield shell and lead–acid battery: what they have in common

1. Step 1. Steel shield shell — is part of — Tunnelling shield. A steel cylinder that holds back the ground.
2. Step 2. Tunnelling shield — is used in — Metro. Metro tunnels were built with shields.
3. Step 3. Metro — is made of — Brick. The vaults of the first tunnels are brick.
4. Step 4. Brick — is material for — Chemical plant. The furnaces and towers are brick.
5. Step 5. Chemical plant — uses — Sulfuric acid. The first chemical plants produced sulfuric acid and soda.
6. Step 6. Sulfuric acid — is used in — Lead–acid battery. The electrolyte.

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