system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why the air expelled from the mouth is smelly while the air expelled from the nose isn’t?
gpt
Your nose fights off stinky germs, while your mouth is happy to have them over for dinner.  
  
EL you're in middle school version:  
  
Your respiratory tract is full of mucus, which has antimicrobial properties. The whole setup of hair, mucus, and cilia is our evolutionary response to containing/eliminating microbial threats in this part of the body.  
  
Then, we have the mouth. Here, we do a whole lot to make bacteria happy. We feed them and give them places to live, so they pay us back by producing bad-smelling waste (among other things). We fight these specifically by brushing/flossing our teeth, using mouthwash, reducing sugar intake, and seeing a dentist regularly. 
 
Edit: Some have pointed out that our mind "filters" out the smell of our nose and the respiratory tract in general. This is true, but breath leaving your nose should not have an offensive smell. If you or an observer notices a stink coming out of your nose, it's likely due to an infection.