Signal post and refrigerator: what they have in common

1. Step 1. Signal post — is part of — Railway semaphore. A post.
2. Step 2. Railway semaphore — is made of — Cast iron. On a cast-iron mast.
3. Step 3. Cast iron — is material for — Ice machine. Gorrie’s machine is cast iron.
4. Step 4. Ice machine — evolved into — Refrigerator. Machine-made cold.

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