system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If muscles can't "push," how do we make our belly expand while taking a deep breath?

All the answers I can find online are some combination of "your belly muscles expand as a natural consequence of your diaphragm moving down" (which doesn't make sense because you aren't putting above atmospheric pressure air into your lungs and we can only relax muscles, not expand them) or "air takes up space so when you breathe in, your body must expand" (which is what your diaphragm is moving for.) Both these mechanisms imply a "forced" draft where air is pushed into your lungs, but my understanding is that we breathe by "induced" draft, where we create low pressure in our bodies and air rushes in.

So why does our belly sometimes move out when we breathe?
gpt
when the lungs are empty, the diaphragm is released and bent upwards. when you contract the diaphragm to suck in air, the diaphragm moves down, meaning it moves to the most contraced point which is kind of straight between where it is connected to your chest bones - so not bent upwards anymore. so it actually moves down and pushes the organs and therefore the belly down and out.