"Yeah. But magic breaks all of it, like how I'm a metamorphmagus so I can change my shape and even get heavier or lighter when I do, and like, where does all that mass go?"

"I don't know, I'm not the expert on magic or laws of physics. What do you mean, mass?"

"When magic isn't around, there are some laws of physics that are called laws of conservation. Like, if a thing weighs two kilograms and you burn it, then its weight plus the weight of all the smoke it produced must remain at two kilograms. And they've tested that, and there are some other laws of physics they discovered that imply that one, so it's pretty set in stone by now... except—"

"—I'm lighter when I'm a girl and I definitely did not breathe out all that mass."

"Maybe the magic can store the mass somewhere? There are Extension Charms...a bag can hold more things than it should."

Back to boy. "Yeah, maybe, but where? And even that, if it doesn't break the laws of physics, bends them like no one's business—like the Platform Nine and Three-Quarters, that makes no sense."

"There are supposed to be laws of Transfiguration. And I think Potions will probably be interesting to you." 

"Yeah! I read the whole Transfiguration book and Charms and I'm almost through Potions!"

"What do you like most so far?"

"Well, Transfiguration, I think, but that may be because of the metamorphmagic thing. I'm a stereotype," he giggles.

"It's a stereotype that they're good at it, I sort of always thought most of them would skip learning it, since they have their own magic for it?"

"Well I can only change myself and changing other stuff is cool. Plus, there's all these rules and stuff and it's very unconstrained when you know what you're doing."

"Is Transfiguration like physics, then? I guessed it was, but I don't really know either one."

"Nnnot quite. Or, at least, not what they teach first years. Physics has more maths."

"Oh. They teach maths when you're this old?"

"Some? Not a lot, most stuff I learnt by myself, but they were teaching us, like, second-degree equations and stuff."

"I don't think we have second-degree equations. Maybe in Arithmancy."

"What's Arithmancy?"

"Magic that has numbers. I think it's like second-degree equations except magic."

"That sounds like fun."

"It's an elective class, you can't take those until your third year. Maybe if you ask nicely." 

"Yeah I'll figure it out."

And at that moment a girl opens the door to their compartment. "Are you two jerks who hate muggles, too?"

"...no, I'm a muggleborn and he's nice."

"Good," she says, and drags her luggage inside, flopping onto a seat.