Crutches and bicycle: what they have in common

1. Step 1. Crutches — evolved into — Wheelchair. Mobility aids.
2. Step 2. Wheelchair — has part — Hub motor. Powered wheelchairs.
3. Step 3. Hub motor — is part of — Electric bicycle. A hub motor or a mid-drive motor.
4. Step 4. Electric bicycle — evolved from — Bicycle. A bicycle with an electric drive.

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