Top spindle and tunnelling shield: what they have in common

1. Step 1. Top spindle — is part of — Spinning top. A rod for spinning it up.
2. Step 2. Spinning top — is made of — Clay. Or shaped from clay.
3. Step 3. Clay — is material for — Brick. Bricks are moulded from clay.
4. Step 4. Brick — is material for — Metro. The vaults of the first tunnels are brick.
5. Step 5. Metro — uses — Tunnelling shield. Metro tunnels were built with shields.

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