system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If the air is 78% nitrogen and 21% oxygen, why do we say we breathe in oxygen but not nitrogen?
gpt
You actually use the oxygen--you absorb it, and emit carbon dioxide. The nitrogen is inert and does not get used by the chemical processes of your body. As much as you breathe in, you also breathe out. But you need the oxygen to live.