system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are some people mouth breathers (myself included) and how can it be fixed?
gpt
There are three causes of mouth breathing: 

**Obstructive** mouth breathing happens when a person can't breath through their nose at all, or can't get enough air through their nose to be comfortable. This can be caused by nasal polyps, septum deviations, sinus inflammating etc.

**Anatomic** mouth breathing is caused by physical defects in a person's face which don't allow their mouth to close comfortably at rest. This can be due to malformation of the lips or jaw.

**Habitual** mouth breathing is not particularly common, but sometimes happens when a person has a temporary nasal obstruction (or frequent temporary obstructions) and gets used to breathing through their mouth.

Mouth breathing is not healthy and can damage your teeth and gums, so you should have a doctor look at your face and nose to determine the cause. If it's just a habit, just try to switch to nose breathing whenever you become aware of your mouth breathing (and perhaps set alarms to remind yourself), and you can change the habit over time.