system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we keep air in space stations breathable?
gpt
It's mostly made from water.

Water as H2O can be split and create 2H2 + O2. That hydrogen is then actually combined with waste CO2 from breathing, creating CH4 (methane) and H2O (more water, fed back into the system).

There's loss everywhere in the process no matter how efficient the reclamation systems are. We send constant supplies of water up to the station.

Similar systems already existed on submarines. Of course, they could provide their own water.