system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes us get a 'Stitch' in our sides when we run too quickly?
gpt
Believe it or not, the real answer is, 'we don't know'! The most common answer is that the jerking nature of running pulls on the ligaments tying the organs in our stomachs to our bones - but this can't be the whole answer, as people get stitches while swimming! 

Another possibility is that as the heart rate increases, the blood pressure into the liver and spleen increases, making them bigger, squeezing some blood vessels partially closed. This causes some places to not get enough oxygen.

A third has to do with breathing. If a runner doesn't breathe deeply, part of their lungs don't get fresh air. As the blood is depleted in oxygen when it gets back to the lungs, the parts of the lungs with stale air in them aren't getting enough oxygen, and getting too much carbon dioxide, causing pain.