"What do you need boy robes for?" inquires Madam Malkins.
"I'm sometimes a boy," the girl explains.
"What?"
"Look—"
"Because magic exists and I'm going to school with a ton of people who've known about it all their lives and there's a whole different world? – And also apparently electricity doesn't work. For some reason."
"It's what I heard. I assume we don't need lightbulbs, though, what with – I dunno, flames of never-going-out?"
"Uh. Magic interferes with it somehow? I'm not sure. You have, uh, computers, right? Computers use electricity? They don't work near magic."