Question

Why do we sneeze and yawn?

Answer 1

Yawning:
when your body lacks supply of oxygen, so it sucks in as much oxygen as possible through a yawn. If you'd notice, you take in so much air when compared to normal breathing. When you feel like yawning, breathe in a lot of air and the sensation would be gone.

Sneezing: your inners of your nose are coated with small hairs, kind of like a filter. It's duty is to filter all the little particles while you inhale. While sometimes, the particles are quite large and cause an irritation inside the nose which stimulates the body to sneeze or expel the said particle out of your nose.

Hope it helps