system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does breathing into a paper bag help with hyperventilating?
gpt
It's clear that your body tries to maintain a certain level of oxygen in your blood. What's surprising is that it doesn't do this by measuring the oxygen in your blood, but by measuring the carbon dioxide instead.

When you hyperventilate, you're sucking in more oxygen, which your body sees and measures as a *drop* in carbon dioxide levels. This actually has some consequences and can affect your blood chemistry, which makes you feel worse and can actually even make you sick. 

Breathing into a paper bag catches your exhaled breath, and makes you reinhale it. A reasonably large fraction of your exhaled breath *is* carbon dioxide; by forcing yourself to reinhale it, you're breathing more CO2 and less O2 than you would by breathing regular air. This brings the levels of both gases in your blood back to normal and helps you feel better.