I didn't spend the time reading the comments to see if this has been said already, but crossing your eyes works but learning to diverge your eyes is better in some situations. It's easier to learn to cross because you can just look at your nose, and you can bring together two images that are further away by crossing than by diverging. But diverging is the way we are normally (when looking across the room say instead of at a book in front of us) so it's more comfortable.