Carabiner and chisel: what they have in common

1. Step 1. Carabiner — has part — Spring. A gate.
2. Step 2. Spring — is part of — Ratchet. The pawl spring.
3. Step 3. Ratchet — is part of — Turnstile. A ratchet.
4. Step 4. Turnstile — is part of — Metro. Turnstiles.
5. Step 5. Metro — uses — Tunnel. Tunnels.
6. Step 6. Tunnel — uses — Chisel. Hezekiah’s Tunnel was cut by hand with chisels.

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