"What is a Parselmouth?" asked Marisa. It was late that night, and as usual the pair were having a secret conversation in the Slytherin common room.
"Someone who can speak to snakes," replied Leo. "Slytherin could do it, so that means everyone's going to think Potter's his heir. Which I don't think is extremely likely, but we definitely can't eliminate him as a suspect."
"Does this mean the monster's a snake?" asked Marisa. "Because that seems very likely given…"
Leo was at first surprised that he hadn't worked this out himself. It seemed so obvious, the natural conclusion to jump to, when Marisa said it, but he hadn't worked it out in months of knowing that Slytherin could speak to snakes.
"I… don't know…" he said slowly. "It does seem likely, you're right, but I can't help thinking it's just too obvious for it to be right… but add it to the list of possibilities."
"It's a very long list," said Marisa, "what with your insistence on not eliminating anything, no matter how unlikely. In fact, it's infinitely long."
Leo rolled his eyes. "It's technically impossible for anything real to be infinite."
"Can you prove that?" asked Marisa, a teasing glint in her eye and a mischievous smile on her face.
Leo glared at her as he quickly tried to think of some kind of proof, but when he began to consider what little he knew of Muggle cosmology he quickly realised that he knew nowhere near enough to prove that the Universe was finite.
He was half-tempted to say something along the lines of "It just is! Because it's ridiculous!" but quickly realised how hypocritical that would make him and the fact that Marisa would definitely point it out, so he sighed and admitted "You win."
Marisa's grin was unbearably smug.