Chalk ledge and sewage treatment plant: 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 a kind of — Lime. Chalk is calcium carbonate, like limestone.
4. Step 4. Lime — is material for — Roman concrete. The binder is lime.
5. Step 5. Roman concrete — evolved into — Concrete. A revival of Roman technology.
6. Step 6. Concrete — is material for — Sewage treatment plant. The tanks are concrete.

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