system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens to your fart when you hold it in and it 'passes'?

Yeah so does it just stay in there or what? How come I don't need to fart after it passes? I know it has to come out again, but where does it go?
gpt
Your colon is an air tight tube.  The ONLY way a fart is getting out is through the anus.  When you hold a fart in, you pushing it back up above the internal anal sphincter.  Pressure below the internal anal sphincter and around the external anal sphincter is that "i have to fart" or "i gotta poop" sensation.  More pressure around the external sphincter leads to greater stretching of the sphincter.  Greater stretching = greater urge.  The fart will eventually come back down, you've just temporarily forced it further up "inside", beyond the physical point in the colon where you can feel the urge to let it out.