"It's definitely why they tell themselves there are gangs of PCP-addled barbecue-fork-wielders roaming the streets after sunset even if they have evidence to the contrary. There isn't a, like, coordinated misinformation campaign going, though. That would require all kinds of nonexistent cooperation."
"Okay, good to know, the Magic Police aren't going to come after me for knowing too much or anything."
"Yeah, they only exist in urban legends, don't worry about it. How'd you find out, anyway?"
This isn't the whole story, but she's keeping the whole story on the down low.
"Really, you'd think books about immortal bitey death-monsters would be a little more interesting, but no."
"Well, their existence is highly interesting. Their contents..." She shrugs. The bell rings. "Meet you and your brother here after school? Can I bring my sister?"
"Works for me," Delilah says, standing up. "The more the merrier."
And after school, there in the library is Bella.
"Oh," he says, looking at Bella. "We have history together, right? Hi."
Bella blinks at him. "Oh! Yeah, we do. Nice to properly meet you, though, I'm Bella, I don't think the history teacher was one of the ones who made me introduce myself to the whole class. And this is my sister Soph."
"Supposedly you are a potential source of lessons in witchcraft?" says Bella.
"Bella's way more interested than me in doing it, but I'm curious," says Soph.
Delilah will reward her curiosity by glowering with nominally less intensity.
"Did I, like, accidentally knock you over in the hall one time and not notice?" Soph asks Delilah.
"I can just about self-teach, given the materials and someone to ask questions of," Bella assures him.
"Del," Neville says with clear exasperation before turning back to Bella. "I meant if you have the aptitude for magic, not if I can teach it."
"About how common is this aptitude? Is it a binary condition or more spectrum-like?"
"Definite spectrum. And I don't know how common it is, honestly. I don't think anyone's ever been able to run controlled trials."