Power amplifier stage and conveyor belt: what they have in common

1. Step 1. Power amplifier stage — is part of — Hi-fi amplifier. Power transistors drive the speakers.
2. Step 2. Hi-fi amplifier — has part — Power supply. A transformer power supply.
3. Step 3. Power supply — is part of — Personal computer. The power supply turns mains power into the voltages the board and drives need.
4. Step 4. Personal computer — has part — Hard disk drive. A hard disk.
5. Step 5. Hard disk drive — has part — Ball bearing. Bearings (later fluid-dynamic ones).
6. Step 6. Ball bearing — is part of — Conveyor belt. The rollers turn on bearings.

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