system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If NASA built a giant, empty storage tank in space, is there a means to pressurize it (with breathable air) without bringing air from Earth.
gpt
Sure. Space is full of stuff that could be made into breathable air.

The simplest way to do it would be to find some ice (probably in the form of a comet) and use electricity (generated with solar or nuclear power) to crack the water into hydrogen and oxygen. The oxygen can be fed into your storage tank for breathing air, and the hydrogen can either be discarded or kept as rocket fuel.

There are also other ways to get oxygen out of certain minerals found in space. However at this point bringing the air with you from Earth is cheaper than finding the materials in space to make air.