system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How breathing into a brown paperbag calms people down?
gpt
It's a little counterintuitive. When you're hyperventilating, the danger is that you're going to run out of carbon dioxide because you're trying to push it out of your system too fast. If the amount of carbon dioxide in your blood gets too low, your blood vessels tighten up, and it's harder to get oxygen to the brain and muscles.

Breathing into a bag makes sure that you're inhaling some of the carbon dioxide back, so you don't run out as easily.