Not long after the disastrous quidditch match against Hufflepuff, snow began to fall at Hogwarts. After classes ended one day a big group of first years went down to the grounds and had a huge snowball fight. Afterwards, faces red from the cold and snow melting on their eyelashes, they went inside for hot chocolate and listened to the Weasley twins tell the story of how they'd unknowingly bewitched snowballs to hit Voldemort's face in their third year.

The lake began to freeze over too, and a few Ravenclaws taught most of the school a simple spell to convert regular shoes into ice skates. This was great fun despite the bumps and bruises one got from falling. It went on until a fourth year fell through the ice and Professor McGonagall told them they were only allowed to skate where the ice was thick enough to hold Hagrid's weight.

On a cold, snowy Saturday when most of the school was at Hogsmeade again, Christina finally found time to go to the library for a purpose unrelated to school work. She had two separate trains of thought she needed to research, the dementors and any possible connection between Sirius Black and her mum. She happened to come across a book titled "Field Guide to Dementors, Boggarts, and Other Nasty Creatures" and took it down from the shelf. The book detailed how dementors sucked the happiness and life out of everything. They were also guards at Azkaban, used to keep morale low. Christina saw now why Black would rather be a wanted criminal than stay in that place. For the worst criminals of all, the dementors would suck their souls out through their mouths, a fate worse than death. Christina turned the page, wondering if the book held any information on how to deal with them. Sure enough, there was a paragraph on something called the patronus charm.

"It's very advanced magic." Said Sam at dinner. "They usually don't start teaching it until you hit NEWT level. Even most seventh years can't do it."

"Maybe they can make an exception for me." Said Christina. "I passed out and got hurt last time one was around. Surely they'd make an exception for students' safety."

"You've seen what goes on in this school." Derrick snorted. "Do you think student safety is anyone's priority? Malfoy got his arm slashed open in care of magical creatures class. That was in a class, with a teacher right there!"

"You can always ask." Said Sam. "But the thing is none of us are very skilled wizards yet. We can barely turn key rings into rubber bands."

"Who do I ask? McGonagall?"

Sam frowned. "I think Lupin's your best bet. He is the one who teaches us to defend ourselves against stuff like this. But McGonagall probably knows too."

"McGonagall just raises an eyebrow and the dementors run away." Said Derrick.

"But I hate to bother Professor Lupin. He always looks so sickly and frail. Plus he was ill again last week. What if this job is stressing him out too much?"

"Maybe you should wait until after the Christmas holidays." Said Sam. "The holidays'll do a lot of good for him. Everyone could use a break."

"Yeah, and besides," Said Derrick, "that's some hard magic. You don't want extra lessons right before Christmas and neither does he.

"Yeah, I'd rather not try to learn a hard spell right now." Said Christina. "It's almost Christmas. I don't want to do anything, but we've still got Snape's essay and we have to turn in our moon charts by the end of the week!" Professor Sinistra had had them chart the phases of the moon since September and wanted them to turn it in as a neat report.

"Oh I need to work on that." Derrick muttered. "I wrote down what phase it was in, but I didn't take any observations like she told us too. I'll either make them up or copy off someone else."

"I'm worried my observations are too repetitive." Said Sam.

""It's just the moon, it's not like you'll see something new and different every night." Said Christina.

"That's true." Said Derrick. "Hey, the moon was full last week, right? That was when I missed class because I had that stomach flu and Madame Pomfrey held me hostage in the hospital wing."

Christina checked her chart. "Yeah, it was full on Tuesday. That was when Lupin was out too."

"That stomach flu must be going around." Said Abigail.

"That's weird. I didn't see Lupin in the hospital wing." Derrick was fiddling with the end of his wand. "But he would've been there, right? He was sick the same time I was. Roger Davies was there puking his guts out and Blaise Zabini was complaining about how his head hurt the whole time, but no Lupin."

"Maybe the teachers go somewhere else when they're sick."

"But why would they?" Said Sam. "Madame Pomfrey's one of the best around, except for maybe St. Mungo's, but people only go there if it's really, really bad."

"Maybe Professor Lupin needs a special doctor for whatever he has." Said Christina.

Christina frowned. "He does seem to be out about once a month. Maybe it's some kind of chronic condition that flares up every now and then."

"That's true." Said Sam. "It might be something that's beyond even Madame Pomfrey's skills."

Derrick was scratching away at his moon chart, writing in the phases and surreptitiously copying off Abigail's observations. He frowned and scratched behind his ear with his quill. "He's been gone pretty consistently right around the full moon. Maybe he's a werewolf."

"Don't be ridiculous." Said Sam.

"Wait, werewolves exist in the wizarding world?" Christina asked.

"They definitely do." Said Derrick. "As a general rule, if you've heard about something in a muggle fairy tale like goblins or trolls or mermaids, they exist in the wizarding world. We've even got centaurs who live in the forbidden forest."

"There's no way Lupin's a werewolf." Said Sam. "They're incredibly dangerous creatures; Dumbledore would never hire one to teach students. It must be something else. A chronic condition, like Christina said."

"Yeah, a chronic condition that flares up regularly every month at the full moon." Derrick snorted.

Christina pulled out her potions book and a roll of parchment and set them on the table. "Look, I don't care whether Professor Lupin's a werewolf or not. I'm still gonna ask him about that dementor charm next term. And now I'm going to start Professor Snape's essay."