system
Given the following post, respond as if you're commenting on Reddit:
human
ELi5 - Do puffed out cheeks when holding breath actually get used?

In other words, if I jump in a pool and fill up my lungs and mouth with the stereotypical chipmunk cheeks full of air, do my lungs use any of that air in my cheeks as like a “reserve”?  Or is it only the air that made it into the lungs that gets used?
gpt
The main issue with this is where do you get the pressure to puff your cheeks? There has to be some air pressure pushing your cheeks outward and it’s not from the outside because then the pressure inside and outside your cheeks would match and they wouldn’t puff out. So that air pressure must be coming from you lungs. If you’re puffing your cheeks, you’re pushing air out of your lungs to do it.
Try this, fill your lungs with as much air as you can, then puff your cheeks, theoretically this is 100% maximum air capacity. But you’ll find that you can pull the cheek air back into your lungs if you try. That’s because it came from your lungs.