Okay, so this isn't quite as good as Reasoning it Out, and it's completely un-beta'd so all mistakes are my own. Still the little beastie grabbed me by the brains and wouldn't let me go until I wrote it out. Just take it with a grain of WTF.

Takes place during Prank Wars and tries to answer the universal Dasey question: Why did Casey choose Derek's shirt to wear to school that day?

Disclaimer: Naught is mine. For soothe. Not even the vague, mis-quoted 10 Things I Hate About You reference or the quote from "Cupid's Revenge."

Cooties

"Sooo," Derek mused as he slid into the seat behind Casey, "one question—why are you wearing my shirt? Didn't Nora have anything drier-fresh enough for you, princess?"

Casey whipped around in her chair, and for a moment the fury and promised violence in her eyes made Derek sit back. But only for a moment. He was Derek Venturi, after all.

"You wan to know why?" she seethed. "Do ya, punk?"

He blinked. "Uh, kinda why I asked that, yeah."

She sucked in a deep breath, her face mottling, and snarled, "Girl cooties!"

A sputtered snort erupted out of him against his will, but really, he couldn't help it! (No matter what Casey's flared nostrils and grinding teeth thought.)

"Cooties?" he choked.

"You heard me!" She pinched the cotton of his I'm an Orangutan t-shirt up by her shoulders in both hands and held it away from her chest. "I'm going to girl-cootie up your shirt so you can never wear it again."

Again with the laughing. And it only seemed to be making her angrier, if that was even possible at this point. Which, when he thought of it that way, was all the more reason to keep doing it. So he laughed. And pointed. And laughed some more as Casey turned murderously red and he imagined steam coming out of her ears.

The bell rang and Mr. Patterson stepped up to the blackboard. "Unless you're discussion the wonders of geometry proofs, all eyes up here."

After she gave Derek one last glare, Casey turned around to face the front, her stringy, messy hair sliding across the top of his desk. She hadn't had time to brush it today, much less spend twenty minutes putting those artful curls in her hair which was naturally poker straight. This pleased him.

As the math teacher started going over the homework proofs they'd had the night before—ones that Casey had done, but Derek… need that sentence even be finished?—Casey participated by raising her hand and asking for clarification on one or two problems while a few of the other students did their homework in class more-or-less surreptitiously. Clearly she was determined to ignore him for the rest of class. And that? Simply not acceptable.

Derek started the next wave of antagonism by simply stretching his long legs out under his desk and through the rear legs of Casey's chair to rest his soles against the front two legs. He then tapped out a rhythm using his toes making Casey try to stomp on his feet in frustration. He then alternated between tugging on little strands of her hair and tickling between her shoulder blades with the eraser of his pencil. But aside from some twitching (and growling, and if that wasn't proof enough that Casey was another species, nothing was) she ignored his efforts.

Plan B.

"Case!" he whisper-shouted. "Hey, Casey. This is important."

She huffed and turned her head slightly to hiss, "What?"

"I noticed something this morning. You talk in your sleep."

She gasped. "I do not!"

"You really do," he insisted. "Who's this Antonio guy, and what was that you wanted him to do with your toes?"

"De-rek!"

"Ms. McDonald," Mr. Patterson interrupted.

Casey gulped and turned guiltily around to face the teacher while Derek did his best to stifle his chuckles.

"Yes, Mr. Patterson?"

"Did you have something you needed to say to the class?"

Derek expected her to shrink back in her chair and pretend nothing happened. That would be the quiet, keener thing to do—not make waves. He forgot for a minute that while Casey might be the second one, "quiet" was definitely not a word best used to describe her.

"Actually, yes, I do," she said, her voice rising to carry to the rest of the class. "Derek is a bottom-feeding jerk. Not to mention a thief and a liar."

"Why, Casey," Derek said, his hand pressed over the place his heart would have been had he not tragically been born without one, "that hurts."

And cue laughter from the audience—or, classmates, whichever.

"Very funny, Mr. Venturi," the teacher said. He opened his mouth again, most likely to give Derek a talking to, so Derek braced himself for impending boredom and reminded himself to try to nod in the right places.

"Mr. Patterson?" Casey interjected into what looked like one of Patterson's more long-winded speeches on classroom etiquette (for which Derek was grateful. Not grateful enough to actually say or, god forbid, do anything, but grateful in a mellow, laid-back, couldn't-care-less kind of way).

"Yes, Casey?"

"Can I switch seats for today?"

Okay, gratitude gone now. She was trying to remove his favorite classwork-distraction technique away from him! Completely unacceptable. Who did this girl think she was?

"And disrupt your academic schedule for the day?" Derek demanded. "Come on, Case, you chose this seat with a systematic, keen-tastic psycho-system with concerns about lighting and acoustics and board-visibility and access to the door so you can get to your next class on time."

"And you chose the seat behind me to bug me," she said. "We all have to live with some deprivation. And we'll both survive the day if I can remove myself from your immediate sphere."

Mr. Patterson looked like he would dearly love to remove both of them from his classroom, but instead he sighed. "You have 35 minutes left to class. Can you hold out?"

Casey, being the good person that she was (a.k.a., a sucker) pouted but agreed to try. ("For the sake of my education.") Patterson shot Derek what he assumed to be a warning look, so he sent him back one of wounded innocence. Were teachers allowed to roll their eyes at students?

Once Mr. Patterson was back at the blackboard, droning on about the newest rule of logic and algorithms or whatever, Derek went back to work bugging his step-sister. It was the only thing he ever put much effort into, besides hockey, and for the same reasons. Both activities demanded complete concentration, ingenuity, finesse, a hint of creativity, and a truck-load of determination.

"Pst! Case! If I call 'Antonio', I only think it's fair to warn him about your snoring."

Another paper ball joined the growing colony in Casey's hair. When he, again, got no response other than the tensing of her shoulders, Derek chose to back off. For now. He was getting bored, and she wasn't responding the way he'd hoped. It was clearly time to up the ante.

One minute slipped by. Then five. Casey was comfortable now, in her groove, working steadily away at the math problems. He let her get comfortable and hopefully she would let her guard down for him. Derek watched her shoulders shake as she erased something on her paper, then bunch together as she concentrated to find the right answer.

The next phase of "Irritate Casey Until She Snaps and Gets Sent to a Lunatic Assylum" came into being as he watched. The material of his shirt wasn't very thick, and he hadn't stolen all of her clothes. And while he hadn't actually done this since he was in Junior High, but he was pretty sure his next plan of attack was just like riding that proverbial bike.

Slowly, checking the rest of the class to make sure no one was watching—especially Patterson—he reached forward. His Orangutan shirt wasn't so big on her that he couldn't see the outline of her bra underneath as she hunched over the desk. Then, carefully, he moved his hand into position—close, but not touching her, not yet. He didn't want her to feel the warmth of his hand and become wary before he struck. No, he needed surprise for this maneuver. If she sensed his purpose too soon, he would have to retreat quickly and try again. And Casey might not let her guard down during this period again. The only other class they had together that day was sociology, and she sat one row over and two desks up. Not really conducive to his plan.

Then, with the speed and agility that made him a god on the ice, Derek pinched the plastic connecting ring between Casey's bra strap and band, pulled back, and let loose.

The snap! came less than a second before the THUNK of Casey's math book connecting with the side of his head. She moved so fast he hadn't had time to do more than flinch back in his chair when he saw her spin around. He hadn't expected her to stand up. He hadn't seen she was armed, either. And he was too busy laughing to care much at the time. (He'd momentarily forgotten that both hockey and Casey-annoyance also required good reflexes.)

"Casey!" Patterson yelled while the rest of the class erupted into laugher and "Ohhh's".

She didn't seem to hear him, or else she ignored him (a teacher!) as she stared down at Derek. He registered through the snickers (and a slight ringing in his ears) that Casey was beyond pissed. Her nose was flared, her eyes were slits of rage (with tears at the corners, which was what really scared him. Honestly.) and her breathing was ragged.

"I…am going…to get you," she promised.

Derek felt he should at least try to look serious. Now if only his face would cooperate. The twitching around his mouth just seemed to provoke her more. She put both hands on his desk and leaned down into his face until he could feel her breath on his mouth. Very distracting. He might have licked his lips, but he was having trouble concentrating.

"I am going to eviscerate you with a nail file," she continued, her gaze snapping back and forth between his eyes. "Then I am going to choke you to death with your own intestines. I am going to make the Spanish Inquisition look like a friggin' tea party with muffins and scones compared to what I do to you!"

"Casey!" Mr. Patterson yelled again for her attention.

By now the tears of rage had built to breaking pointing Casey's eyes (which was the only reason Derek had stopped laughing and started looking a little panicy). Mr. Patterson must have seen it because he just shook his head and gestured to the far side of the room.

"Go trade seats with Evan for the day."

"Thank you," Casey murmured with as much dignity as she could muster under the circumstances.

While she gathered up her books and purse, shooting death threats at Derek with her eyes all the while, Mr. Patterson shook his head.

"Venturi, let me give you a piece of advice my mother like to give my brother and me when we annoyed her: 'The fool that willingly provokes a woman has made himself another evil angel and new hell to which all other torments are but mere pastime.'"

Derek shrugged and watched Casey walk over to where Evan Mears usually sat and take her seat. (As Evan was currently settling into Casey's usual place.) "That's probably true, Mr. Patterson. But what greater thrill is there than staring down the mouth of Hell and daring the devil to come get you?"

He said it quietly, though. Casey just might come get him, and he'd really like to live to see legal drinking age. And with the tears, it would just be messy. Plus, she'd probably be expelled, and he'd get the blame for it. More trouble than it was worth all around.

Mr. Patterson opened his mouth to retort, but then obviously thought better of whatever he was going to say and changed it to, "One of these days that girl is going to bitch-slap you. And when she does, not only will I not stop her, I'm going to laugh."

Those in hearing distance did laugh, and Derek noticed Casey shooting him another hostile look. He sent a smirk back at her, then he leaned forward and tapped Evan on the shoulder. "Dude, you happen to know how to de-girl-cootie a shirt? If not, I'm gonna hafta burn that thing."