system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why we are focusing on getting to Mars instead of building a permanent structure on the moon.

Given the technology and effort of building the ISS, and given the long trip time of going to Mars, why haven't we attempted to build a self-sustaining structure on the moon?
gpt
Because until recently it wasn't known that the Moon has water which meant that it was believed that things like oxygen would have to be shipped constantly to a Moon colony to keep it running.

Plus it'd be expensive.  

Plus Mars is a much better destination.  It has the same amount of land as the Earth (no oceans you see) and could possibly be terraformed given enough time.