Chalk ledge and railway semaphore: what they have in common

1. Step 1. Chalk ledge — is part of — Blackboard. A tray.
2. Step 2. Blackboard — uses — Chalk. People write with chalk.
3. Step 3. Chalk — is material for — Tablet (medicine). Tablet fillers are chalk and starch.
4. Step 4. Tablet (medicine) — uses — Hydraulic press. Tablets are pressed.
5. Step 5. Hydraulic press — is made of — Cast iron. Bramah’s press is cast iron.
6. Step 6. Cast iron — is material for — Railway semaphore. On a cast-iron mast.

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