Bell gong and LIGO: what they have in common

1. Step 1. Bell gong — is part of — School bell. A metal cup resonator.
2. Step 2. School bell — has part — Electromagnet. An electromagnet strikes the hammer.
3. Step 3. Electromagnet — is part of — Burglar alarm. A bell.
4. Step 4. Burglar alarm — uses — Smoke detector. A security system often includes smoke detectors too.
5. Step 5. Smoke detector — has part — Photodiode. An optical scattering sensor.
6. Step 6. Photodiode — is part of — LIGO. A photodetector of the interference.

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