"But... how do they naturally occur? In my world all stuff is inclined to go towards other stuff, more so if it's big or nearby, and so planets are stuff that got near other stuff in a ball shape because this is all taking place in a vacuum."

"...Your world is in a vacuum? That sounds uncomfortable."

Giggle. "Air is stuff, it goes toward the planet. It's not very dense stuff, so it's on top of the rocks, mostly. We don't go in the part with vacuum."

"That makes rather more sense. Yeah, okay, I don't know of a meta-rule like that governing the origin and placement of planets. And there aren't parts with vacuum, there's just more air. So your way seems tidier but not necessarily more convenient for the residents, at least not if the residents can fly. I imagine it would be an unpleasant surprise to be on your way up and then suddenly whoops no more breathing for you."

"The air gets thinner as you go up, you can notice that climbing mountains. And it's cold. You'd notice."

"Well, fair enough, but still - as long as I'm designing a hypothetical world, might as well make it so people can breathe most everywhere, right?"

"How does your sun work, in - air?"

"It's a large ball of fire, what about yours?"

"It's basically a ball of fire but I'd expect it to burn any air near it."

"Maybe it burns the air that's near it but the air near that burns somewhat less of its neighbouring air and so forth and then eventually the outlying bits aren't on fire anymore and you have the edge of the sun. I have no idea if that's what's happening, it just seemed vaguely plausible."

"That sounds wrong but I don't know that much about my physics to tell you why."

"Well - you have fire that's not the sun, presumably, and it doesn't burn everything in its path, so there's got to be some limitations to 'fire sets neighbouring air on fire'..."

"The sun is many times bigger than the planet and hotter than any fire you could make on Earth, I think."

"Maybe ours isn't as hot and that's the trick, or maybe fire works differently here the same way down does."

"Yeah, could be either or both."

Shrug. "I wonder what else is like that - differences between our worlds that aren't obvious until you run into them..."

"I mean, the physics thing will have all kinds of implications."

"Like what?"

"Maybe electricity wouldn't work here, or airplanes wouldn't. Mountain climbing must be easier if the air doesn't get thinner - maybe mountains don't usually have snow on top -"

"Many of them do!"

"Even in places that aren't cold?"

"I don't remember offhand, but we could probably manage to look it up..."

"It's not terribly important. I think at home it has to do with the thinner air though."

"Huh. I'm now tremendously curious what all the other worlds are like..."

"You can see why I wondered why there wasn't more contact with them!"