Tracer arm and pneumatic tube: what they have in common

1. Step 1. Tracer arm — is part of — Planimeter. A stylus traces the outline.
2. Step 2. Planimeter — has part — Wheel. A measuring wheel computes the area.
3. Step 3. Wheel — is part of — Steam locomotive. The steam engine turns the driving wheels.
4. Step 4. Steam locomotive — is used in — Postal service. Mail trains.
5. Step 5. Postal service — includes — Pneumatic tube. The post.

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