Trigger and air conditioner: what they have in common

1. Step 1. Trigger — is part of — Musket. A trigger.
2. Step 2. Musket — has part — Flintlock. A flintlock.
3. Step 3. Flintlock — uses — Friction. A spark from a strike of flint.
4. Step 4. Friction — is used in — Air brake. Brake shoes slow the wheels through friction.
5. Step 5. Air brake — has part — Compressor. A compressor pumps the air.
6. Step 6. Compressor — is part of — Air conditioner. A compressor.

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