Sleeper and radio: what they have in common

1. Step 1. Sleeper — is made of — Concrete. Today sleepers are more often concrete than wood.
2. Step 2. Concrete — is material for — Airport. The runways are concrete.
3. Step 3. Airport — uses — Air traffic control. Air traffic control.
4. Step 4. Air traffic control — uses — Radio. Radio communication.

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