Folding legs and tunnelling shield: what they have in common

1. Step 1. Folding legs — is part of — Ironing board. Scissor supports.
2. Step 2. Ironing board — is made of — Cotton. Covered with a cotton cover.
3. Step 3. Cotton — is material for — Wright Flyer. The covering is muslin.
4. Step 4. Wright Flyer — uses — Railway track. The Flyer had no wheels: it took off along a wooden rail.
5. Step 5. Railway track — is used in — Metro. It runs on rails.
6. Step 6. Metro — uses — Tunnelling shield. Metro tunnels were built with shields.

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