Photometer and railway track: what they have in common

1. Step 1. Photometer — uses — Reflection of light. Brightness is judged from reflected light.
2. Step 2. Reflection of light — is used in — Periscope. Reflection.
3. Step 3. Periscope — is part of — Submarine. A periscope.
4. Step 4. Submarine — has part — Ballast tank. Ballast tanks.
5. Step 5. Ballast tank — is made of — Wrought iron. The tanks are iron.
6. Step 6. Wrought iron — is material for — Railway track. Then — wrought iron.

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