Lens and electric generator: what they have in common

1. Step 1. Lens — is part of — Otoscope. A lens.
2. Step 2. Otoscope — has part — LED. Illumination.
3. Step 3. LED — is part of — Bike light with dynamo. An LED.
4. Step 4. Bike light with dynamo — has part — Electric generator. A dynamo.

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