"Professor?" Harry asked tentatively. Professor Lupin had not said anything for a few instants too many.
Professor Lupin half-opened his mouth, then closed it. How should he discourage them from accepting Dumbledore's offer? At first glance, proposing to have a teacher personally tutor them on the Patronus Charm seemed innocent enough; it might even be seen as generous. However, teaching these two exactly what they wanted would only lead them to seek out more trouble than they could actually deal with.
To serve as an analogy, most cases of drowning happened to people who knew how to swim, since they'd put themselves in situations where they could drown much more often than people who couldn't swim.
And there was another thing. Lupin suspected that Professor Dumbledore had some doubts about Oleandra. The headmaster hated Dementors more than anything else in this world, and the Dark Wizards who would make use of them.
It had to be known that in most cases, evil Wizards couldn't use the Patronus Charm. Best-case scenario, nothing would happen. Worst-case scenario, their magic would rebound; instead of emitting a Guardian Spirit, the wand would spit out a host of maggots, which would devour the caster. (Evil Wizards could still cast the Patronus Charm successfully, provided that they were seriously deluded and thought themselves good.)
"Is there no way I can convince you two to stay away from those creatures?" Professor Lupin said tiredly.
"I don't need any convincing," Harry said quietly. "I know I'm weak. I just… don't want to hear my mum dying again. Not ever."
Or did he?
"You're not weak," Professor Lupin said sharply. "It has nothing to do with that. You're both more affected than most, because you have witnessed horrors others should count themselves lucky they haven't experienced."
Even Oleandra's magically protected soul wasn't immune from their influence. When it came down to it, Mannaz would shield her from critical damage such as the Kiss, but Dementors didn't only attack the soul. Mind, body, soul. All three were connected, and any problem with one would affect the two others.
"Nothing I've thrown at them works," Oleandra said, gritting her teeth. "I've hit them with the force of a speeding car and even with a lightning bolt, but it all goes through them."
"I've heard about that," Lupin said. "There's a reason why Wizards don't often use lightning magic. You know how Muggle electronics stop working in Hogwarts, yes?"
Oleandra nodded.
"Electromagnetism is one of the four fundamental forces of science," she said. "The laws of science get all twisted when too much magic's in the air."
"Interesting," Professor Lupin noted. "I wouldn't have thought that a noble would know that much… Well, I'm not exactly an expert on Muggle science, but I think you've got the gist of it. At any rate, the opposite is also true. Wherever science is strong, mystery diminishes. Near Chernobyl, for instance. Or during thunderstorms."
He paused for a moment.
"And there's also the fact that lightning is notoriously capricious," Professor Lupin added, "you never know if your spell will rebound on you. There's a reason why Thunderbirds are known as flying calamities."
So that was why the Hagal rune had come with a warning label when she had first learned her magic from the stars, Oleandra realized. Maybe using the rune as portable air-conditioning device hadn't been her brightest idea. Now that she thought about it, in her second year, she had summoned electricity between her fingers while standing in a puddle of water back when Mrs. Norris had been petrified. Now that had been especially stupid! And she had flown into a thunderstorm carrying an umbrella! Did she have a death wish???
Meanwhile, as Oleandra was decrying her own stupidity, Professor Lupin noticed Harry's expression. The poor boy had bared his heart, and in return he'd received a science lesson. Lupin sighed.
"All right, I get the message," Professor Lupin said. "If you want the means to protect yourselves from Dementors, then I will personally teach you the Patronus Charm."
"The Patronus Charm?" Harry echoed.
"I know all about it," Oleandra said confidently. "Um, but you might want to tell Harry. For his benefit."
"Of course," said Professor Lupin with a twinkle in his eye. "But let me warn you right now; this spell is highly advanced white magic, well beyond the Ordinary Wizarding Level. Don't be disappointed if you don't succeed at first. Many highly qualified Wizards can't conjure a Patronus."
"What's a Patronus?" Harry asked nervously.
"Well, to put it simply, the spell conjures an Anti-Dementor; the Patronus Guardian Spirit. It's a mass of pure, positive emotions— everything that a Dementor likes to feed on. However, as it's not a real person, the Dementor can't hurt it, which means they'll feed off of it until they get indigestion. If they're filled beyond capacity, then just like the Boggart earlier in the year, they'll pop. After a while, they'll be reborn."
"What's it look like?" Harry asked curiously. "The Guardian Spirit, I mean."
"To answer your question, Harry," Professor Lupin said, "each Patronus takes the form of an animal. Each one is different, it depends on the Wizard."
"And how do you conjure one?" Harry followed up.
"You'll need to picture a single, very happy memory," Professor Lupin explained. "And once you've got it, you'll need to recite the incantation— Expecto Patronum!"
"Expecto Patronum, Expecto Patronum—" Oleandra heard Harry say under his breath.
"Concentrate," Professor Lupin encouraged him.
As Harry was repeating those words like a mantra, a wisp of silvery gas escaped from his wand and floated a bit in the air, before disappearing.
"Did you see that?" Harry exclaimed. "Something happened!"
"Very good," said Professor Lupin with a smile. "That's the first form the charm takes; the Incorporeal Patronus!"
"My turn," said Oleandra eagerly.
She pictured herself with her family, shopping happily on Diagon Alley after she had discovered she was magic. Actually, wasn't the moment she received her Hogwarts acceptance letter a better memory? Whoops, that was two things. The charm would only work with a single happy memory.
"Expecto patronum!" Oleandra said with determination, once she had settled on the latter.
Instead of gas, a viscous silvery liquid cascaded out of the tip of her wand and pooled at her feet.
"Haha!" Oleandra shouted with glee. "I've gone straight to the second stage; The Liquid Patronus! I must be a genius!"
Gas->Liquid->Solid!
"There's no such thing," said Professor Lupin in wonder. "What in the world is going on with your Incorporeal Patronus?"