Turnstile and hammer: what they have in common

1. Step 1. Turnstile — is part of — Metro. Turnstiles.
2. Step 2. Metro — uses — Tunnelling shield. Metro tunnels were built with shields.
3. Step 3. Tunnelling shield — uses — Hydraulic cylinder. The shield was pushed forward by jacks.
4. Step 4. Hydraulic cylinder — is part of — Forging press. Working cylinders.
5. Step 5. Forging press — evolved from — Steam hammer. Pressing instead of striking.
6. Step 6. Steam hammer — evolved from — Hammer. A steam hammer is a hammer lifted by steam, used to forge huge parts.

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