system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens when someone breathes in a brown paper bag and why is it supposed to calm them down?
gpt
One thing that happens when you panic is that you hyperventilate. This means you are breathing too fast and you over oxygenate your system, which is great if you are having to flee a tiger and need the oxygen to fuel your muscles but bad when you are having a panic attack and have no outlet for the nervous energy. 

What breathing in the bag does in slightly increase the concentration of carbon dioxide that you breathe thus reducing oxygen intake slightly, and it gives you a visual representation of your breath rate as the bag inflates allowing you focus on something to slow the rate of breathing.