Answer:
When you feel sleepy, you yawn. When you feel bored, you yawn. The kid next to you yawns. You yawn, too--even if you are wide awake and having a good time. So why are you yawning? Let's check it out. Your body needs oxygen. You get oxygen by breathing in air. Your body also needs to get rid of carbon dioxide.
When you breathe out, this carbon dioxide is carried out of your body. Your brain makes sure you have the right amount of oxygen and carbon dioxide in your body. If you have too much carbon dioxide in your body, your brain tells your mouth to open. It tells your throat to tighten. And it tells your chest to take a deep breath. In other words, your brain makes you yawn. Oxygen rushes in. Carbon dioxide rushes out.