Not-giggle.

"Is Mechahitler a Muggle celebrity?"

Both of them—

—start giggling helplessly.

After a second: "No, Hitler was an evil despot a few decades ago, and 'mecha' just means 'robot suit.'"

"Oh. That's not as funny. We had an evil despot a few decades ago, too, and then he came back. That was the war you just missed."

"Well the joke is a bit tasteless but it's the kind of thing a kid could come up with."

"...maybe not if we just missed it, we just missed a war?"

"He-Who-Must-Not-Be-Named looked like he was dead. Turns out he wasn't, or he had a way back. We don't know which. He took over the government, two years ago. They were hunting down muggleborn and halfblood witches and wizards."

"Wow is everyone here horrible?"

"The Dark Lord and his followers, and they're mostly dead."

"Some people were horrible in the other compartment."

"What house? Could you tell?" 

"Dunno, didn't ask."

"You'll mostly get that from Slytherins. Sadde is going to fix that."

"How?"

"Conspiracy of firsties!"

"Do you think the other houses need fixing, too, or just the one who was too friendly with the crazy overlord."

"Probably, but their problems are likely not as huge."

"What about you, Willow, anything you're going to take over?"

"I dunno, I'm not actually a Slytherin."

"What do you think you'll be? I'm going to be a Gryffindor."

"Ravenclaw! Or Hufflepuff."

"I'm a Gryffindor, and Sadde thinks his other friend is going to a Hufflepuff, so you can be our Ravenclaw friend, and that gives us all four!"

"Ooh, awesome!"

"See, I'm so good at getting friends I don't even have to do anything."

"Well, if you were a bad Slytherin, we couldn't be friends, but you're going to make more good Slytherins, so that makes you kind of the best Slytherin."