Tok blinked, tilting her head. "Well. You go up a really tall mountain, or fly up high in't sky, air gets thin. Not enough of it. Stands t'reason, you get far enough away from't ground, there's no air left."

She hadn't been to space before, but this was sound logic based on observational evidence and a few testimonies from very unintelligent mages who'd teleported far higher than they should've.