Stretcher poles and electric motor: what they have in common

1. Step 1. Stretcher poles — is part of — Stretcher. Two lengthwise handles.
2. Step 2. Stretcher — is part of — Ambulance. A stretcher.
3. Step 3. Ambulance — evolved from — Truck. A special-purpose vehicle.
4. Step 4. Truck — has part — Air brake. Air brakes, as on trains.
5. Step 5. Air brake — is part of — Diesel locomotive. Air brakes.
6. Step 6. Diesel locomotive — has part — Electric motor. Traction motors turn the wheels.

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