File and pneumatic tube: what they have in common

1. Step 1. File — uses — Friction. Material is removed by the friction of the teeth.
2. Step 2. Friction — is used in — Air brake. Brake shoes slow the wheels through friction.
3. Step 3. Air brake — uses — Compressed air. The brake is controlled by air pressure in the train pipe.
4. Step 4. Compressed air — is used in — Pneumatic tube. Air pressure.

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