Sprocket and personal computer: what they have in common

1. Step 1. Sprocket — is part of — Continuous track. The drive sprocket engages the track and pulls it round — just as a bicycle sprocket pulls the chain.
2. Step 2. Continuous track — has part — Road wheel. Road wheels roll along the track and carry the vehicle’s weight.
3. Step 3. Road wheel — has part — Ball bearing. The road wheels turn on bearings.
4. Step 4. Ball bearing — is part of — Hard disk drive. Bearings (later fluid-dynamic ones).
5. Step 5. Hard disk drive — is part of — Personal computer. A hard disk.

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