Crystal radio and tank: what they have in common

1. Step 1. Crystal radio — has part — Antenna. An antenna.
2. Step 2. Antenna — is part of — Radar. An antenna emits and receives.
3. Step 3. Radar — works like — Sonar. The same echolocation principle: sound in water instead of radio waves.
4. Step 4. Sonar — is part of — Submarine. Underwater acoustics.
5. Step 5. Submarine — has part — Periscope. A periscope.
6. Step 6. Periscope — is part of — Tank. Observation devices.

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