Lightning Dust glanced briefly down, and this time she didn’t seem confused. “One closest to the Crystal Empire is as far as unicorns are supposed to go. Past that is for earth ponies… they’re tougher, so they can go further. The last one is for pegasi. Past that line, and it’s almost impossible to fly. That close to the Nibiru, even strong flyers can tumble out of the air like foals on their first day. Not much of a point to go up that far normally, though. Unless you’re in it for the view. It’s too cold to grow anything, and the rocks are so awful even diamond dogs don’t want to live in them.” She stopped, looking abruptly away.