system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 why it is easy to cross my eyes, but impossible to move them apart from each other?
gpt
Crossing your eyes is important in order to focus your eyes on an object that is very close.

"Splitting" your eyes isn't something that your brain has ever needed to do (there is never a case where an object can be focused on by "splitting" the eyes), so the muscles aren't trained on how to do that.