Rubber crutch tip and headphones: what they have in common

1. Step 1. Rubber crutch tip — is part of — Crutches. It prevents slipping.
2. Step 2. Crutches — evolved into — Wheelchair. Mobility aids.
3. Step 3. Wheelchair — has part — Hub motor. Powered wheelchairs.
4. Step 4. Hub motor — uses — Electromagnetism. A hub motor is an electric motor.
5. Step 5. Electromagnetism — is used in — Loudspeaker. A coil carrying current in the field of a permanent magnet moves the diaphragm.
6. Step 6. Loudspeaker — is part of — Headphones. Miniature drivers.

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