Grammar check: Your sentence is almost correct. You can say either:
"Which tube lets air into the lungs?" or "What tube carries air into the lungs?"
Both are good. You do not need the word "the" before "air" — saying just "air" sounds more natural.
Answer for a 7-year-old: The main tube that lets air into your lungs is called the trachea. People sometimes call it the windpipe.
How air travels (step by step):strong>
- First air goes in through your nose or mouth.
- It then goes down the trachea (windpipe).
- The trachea splits into two smaller tubes called bronchi — one goes to each lung.
- Inside the lungs the air travels into tiny air sacs called alveoli, where your body takes the oxygen it needs.
Think of the trachea as a big slide that leads to two smaller slides (the bronchi) that go into each lung.