Chalk and sonar: what they have in common

1. Step 1. Chalk — is a kind of — Lime. Chalk is calcium carbonate, like limestone.
2. Step 2. Lime — is material for — Roman concrete. The binder is lime.
3. Step 3. Roman concrete — evolved into — Concrete. A revival of Roman technology.
4. Step 4. Concrete — is material for — Airport. The runways are concrete.
5. Step 5. Airport — uses — Radar. Radars.
6. Step 6. Radar — works like — Sonar. The same echolocation principle: sound in water instead of radio waves.

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