I used to have a model of breathing that went something like this: when breathing in, the lungs somehow get bigger, creating lower air pressure inside the lungs causing air to flow in. Then when breathing out the lungs get smaller, creating higher air pressure inside the lungs and causing air to flow out. How do the lungs get bigger and smaller? Eventually I learned that there's a muscle called the diaphragm that is attached to the bottom of the lungs (??) that pulls or pushes the lungs. If I keep my nose plugged but my mouth open, the air will travel through my mouth. If I keep my mouth closed but my nose open, the air will travel through my nostrils. So far, so good.

Then a few days ago, I noticed that if I keep both my nose and mouth open, I could choose to breathe in solely through one or the other. This... doesn't make sense, according to the model. The model would predict that the air just flows through both pathways, maybe preferentially going through the mouth since that seems like the larger pathway.

So something is clearly wrong with how I think about breathing. Is there some sort of further switch inside that blocks one of the pathways? Does the nose or the mouth contain variable-size cavities that can control air pressure to direct the flow? I still have no idea. I'm eventually going to look it up, but I might think about this for a little bit longer (or maybe someone here will tell me).

I thought this was a pretty interesting example of how the explanations you hear about seemingly-basic things are easy to accept but don't make sense on further reflection. But it's hard to notice the flaw too. In my case, after a recent ENT visit where I was told my nasal passages are inflamed, I've been putting more effort into consciously breathing through my nose. Then one day I woke up and as soon as I woke up I did something like consciously breathe through my nose with mouth closed, and then somehow I opened my mouth but then still tried to breathe through my nose (or maybe it was that I noticed I was breathing through my mouth, and since I was still waking up I didn't bother to close my mouth and just tried to breathe through my nose with my mouth open), and was surprised this was even possible.