- Someone plays the flute
- If a flute is played, then the air in the flute will vibrate
- Therefore, the air in the flute will vibrate when the flute is played

- The air in the flute will vibrate when the flute is played
- Vibrations are a source of sound
- Hence, the vibrations of air in the flute are a source of sound when the flute is played

Hence, the vibrations of air in the flute are a source of sound when the flute is played
Question
When someone plays the flute, sound is produced because the air in the flute is

Context
- Someone plays the flute 
- Vibrations are a source of sound 
- If a flute is played, then the air in the flute will vibrate