Crutches and knife: what they have in common

1. Step 1. Crutches — evolved into — Wheelchair. Mobility aids.
2. Step 2. Wheelchair — has part — Ball bearing. Bearings.
3. Step 3. Ball bearing — is part of — Crankshaft. The main journals of the shaft are bearings (in combustion engines usually plain bearings).
4. Step 4. Crankshaft — is a kind of — Crank. A crankshaft is several cranks on one shaft.
5. Step 5. Crank — is part of — Meat grinder. A handle.
6. Step 6. Meat grinder — has part — Knife. A cross-shaped blade.

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