"This is your fault, you know," Kelsey hissed. "We could be relaxing at home for our vacation, but no..."

"Hey, I didn't ask for this," Matti reminded her. "It was a surprise to me, too."

Kelsey rolled her eyes in the way that always made Matti feel stupid, even though she knew that, technically, she was probably smarter than her older sister. It was just one of Kelsey's special skills, and this year, after everything that had happened, it was super effective, making Matti squirm and blush before the other girl had even said anything else.

"We're only doing this because you used to like trains," Kelsey told her. That was probably true, actually. Matti didn't recall expressing interest in this, ever, and it was obvious from Kelsey's reaction that she didn't have any. She couldn't think of any other reason why their parents would suddenly decide this was how they ought to spend a day of Christmas vacation.

"So, you agree, then?" Kelsey pushed, taking Matti's silence, before and after the question, as solidarity. "Come on."

She grabbed Matti by the hand, dragging her to the living room. "Listen," she spoke for both of them, "Mom, dad. It's nice that you want to do this together, but, seriously, there has to be something else we could do. This is for babies who still believe in Santa, and you know we aren't that anymore, right?"

She looked to Matti for backup, but her younger sister had none to provide. Already, hearing the word 'baby' coming out of Kelsey's mouth made her feel weird enough. Asking her about Santa didn't help.

Last year, Matti would have had no problem denying his existence. Most likely, that was still the case, at least here, but she was less certain, about a lot of things. She'd read all those stories of Sierra encountering him, after all, and then seen Sierra in the flesh.

Her cheeks warmed, envisioning all the punishments Sierra had been given by Santa, all the times she'd gotten herself regressed, turned from a young woman into a helpless toddler. That wasn't going to happen to her, of course. The Santa they were going to see wouldn't be the real one, on the slight chance they actually had one in this dimension in the first place, but it was hard not to imagine coming home, transformed into a chubby little baby, knowing there was no way to change back until next Christmas. That would probably cheer Kelsey up, too. Matti was sure she'd prefer it that way, having a little sister who she didn't have to share a school with, who she could go to college and leave behind, trapped in her crib and playpen, stuck in diapers, who, when she was home on break, she could dress up however she wanted.

"Seriously, Matti?" Kelsey huffed, sounding betrayed that Matti hadn't followed through on the promise she hadn't actually made in the first place.

Matti looked from her parents to her sister, giving a tiny shrug. "It might be cute," she said.

"You have been acting so weird lately," Kelsey said, annoyed, as they went back to their rooms. "Something is up with you, I know it."

If only she knew the half of it, Matti thought. There was no way Matti was going to be the one to tell her, however, not if she could avoid it. Unfortunately, there were plenty of other people who might, but now that they were home for the holidays, that was a worry for another time.

At the very least, as a cheerleader, Kelsey was skilled at pretending to be enthusiastic about things she didn't particularly care about, and, whether she wanted to be there herself or not, she didn't let her surly attitude show, so she wouldn't ruin the experience for the kids who were there.

And she looked pretty festive in the tiny red dress she'd picked out, even if Matti was baffled by how she could wear it without freezing. Then again, it was no shorter than her cheerleading uniform, and she and her squad wore those out in the cold all the time for games. Matti definitely felt like the younger sister next to her, wearing her overalls and red sweater.

"You're not even going to dress up?" Kelsey sniffed, eyeing her. "This is your day."

Matti frowned, glancing down at herself. "I am dressed up," she said. "See, it's red," she plucked at the sweater's oversized sleeves. "It's festive!"

Kelsey rolled her eyes, though not nearly as hard as she did when they arrived, and turned the corner, Matti getting her first glimpse of the train, sitting there on the track, silver and gleaming and shining in the sun, making her gasp at the beauty of it, of seeing it up close, and knowing she was going to get to ride on it. "You are such a baby," Kelsey whispered. "I don't know why I was surprised you'd want to meet Santa."

"I don't!" Matti glared at her, speaking under her breath. "But, you know, they'd already gotten the tickets, and I didn't know if they were refundable, and..."

"What are you two talking about?" their mother asked.

"Nothing," they replied in unison.

Despite her insistence that it wasn't that big a deal to her anymore, Matti was still shaking a little as their father handed over their tickets, the attendant smiling at them and saying, "Welcome to the Polar Express!" before stepping aside to let them step onboard. Considering how much Matti had been into trains as a kid, and how many toys she'd had, how many hours she'd spent in her room putting together tracks, they had, of course, taken her on them before, but it had been quite a while. That first step took her back instantly, and it was all she could do to prevent herself from racing on ahead to get a look at all of the cars. If Kelsey hadn't been there, judging her, she might have. They had it ready for the holidays, too, with garlands and tinsel and old-timey cardboard decorations.

"Well, where do you two want to sit?" their father asked.

"Go on, you might as well choose," Kelsey pushed Matti forward. Matti took the opportunity to wander to almost the end of the passenger car, taking in as much as she could, before stopping, sliding into one of the seats and staring at the window, practically bouncing in place as she stared out, saw the track stretching ahead of them.

They were greeted with candy canes and hot chocolate while they waited for everyone to board, Christmas carols playing over the speakers, overshadowed now and then by the sound of the whistle blowing, making Matti more excited each time until, finally, they started to move, lurching forward.

"Guess it's too late to get off now," Matti could barely hear Kelsey mumble, but she was beyond caring at that point.

"I'm going to go to the bathroom," she chirped, making her mom slide off the bench so she could get out, the sensation of the train moving beneath her feet bringing back old memories, feeling both strange and comforting. It was similar, she supposed, to walking down the aisle of an airplane, yet, to her, it was completely different, and more thrilling, to have the track beneath her, just like the tracks she'd put together as a child.

She took her time, though not because she felt unstable, just so she could savor it longer. She didn't really need to pee that badly, but she hadn't wanted to wait any longer to look at the train car again as she passed back through. Besides, if she went now, that would give her bladder more of a chance to fill back up so she might have to go again before they left.

She was almost to the bathroom door when one of them opened, and she paused, not knowing if she should step to the side to let the occupant past her, or hurry past to get out of the way completely. Instead, as they stepped out, she froze completely, staring up at him.

It was Santa. Matti couldn't recall the last time she'd gone to see a Santa at the mall, or anywhere else, but she was certain she'd never seen one that looked as real as this. He was huge, his beard obviously not a fake, and his face seemed jolly, even before he glanced down and saw her there, gawking at him.

"Ho ho ho," he smiled. "How are you doing, dear?"

Matti blushed as red as his suit. Was it the way he was talking to her, or her thoughts of those Christmas Sierra stories, that was making her feel like such a little kid, facing him? She knew it was silly, that there was no way he was the real thing, that even if there was such a thing, there was no way she'd happen to run into him here, of all places, and yet, there was a part of her that couldn't help wondering. It didn't help that she was too flustered to speak for a moment as she stood there in her overalls.

"I-I'm good," she said finally. "H-How are you?"

He was amused by that, whether it was her nervous tone or her attempt to sound casual, to respond to him like he wasn't dressed as a mythical being. "Ho ho ho!" he chortled. "I'm wonderful!"

Matti nodded, feeling more and more stupid by the moment. "Cool," was all she could think to say.

"I know what you want for Christmas," Santa winked at her.

Matti's eyes widened. Instantly, she was taken back to a couple weeks before, when she was still at school, to when she was being forced to lie on the floor of a living room, diaper on full display beneath her shirt and fluffy tutu skirt as she wrote out a letter to Santa in crayon, then had to toddle out to the mailbox in the same outfit to mail it.

The memory of that humiliation was enough to make her feel her undies warm slightly, even without recalling what she'd been told to write in the letter, or the thought of anyone actually reading it, much less having the person who did standing right in front of her, smiling down at her.

"Y-You do?" she gulped.

"Oh, I certainly do," he confirmed. "In fact..." He reached into the pocket of his coat, pulling out one white-gloved hand, blowing something glittery at her.

"H-Hey!" she coughed, rubbing her eyes. As soon as she moved her hand, Santa was gone. She frowned, confused, not knowing where he could have possibly gone, since there hadn't been room for someone his size to slide past her without her moving, and there was nowhere further ahead, where she was looking, that he could be hiding.

There wasn't much she could do except shrug, starting to turn around before she remembered why she was here in the first place. She stepped forward, towards the bathroom, only to find it was locked.

That was all right, she thought. She didn't have to go that badly. There was a seat just beyond the door, for people to wait on, so she stepped forward towards it, surprised at how difficult it was, wondering if the train had sped up significantly since she'd walked to the bathroom, and sat.

And that was when she heard the crinkle, and felt the soft, cushy padding beneath her bottom. "No way," she squeaked, raising a hand to her throat at the realization of how high and squeaky her voice was.

Even that revealed a new horror, her fingers looking tiny, and pudgy. "No, no, no!" she whimpered, almost too scared to look further, but she already knew what was happening. Her legs were short enough as it was, probably wouldn't quite have hit the floor, but she could tell they were much further away than they should have been, able to swing even more freely, though, each time they did, she could hear another crinkle.

"No," she nibbled her bottom lip, when she finally did take a glimpse. Santa had turned her into a toddler! "Santa!" she squealed, wrinkling her nose at the sight of the pink leotard she was wearing, the fluffy, pink tutu, the ballet slippers. He had read her letter, it seemed, and her impassioned plea to give her the chance to become the best ballerina in the world, even if that meant making her little again so she could start taking classes then, to give herself the best shot. It had been, by the end, quite convincing. Not because that was something Matti wanted, of course, or had ever been interested in, but because she'd had to read out each draft, and, if it wasn't good enough, start over from the beginning.

Apparently, she thought to herself, it had been enough to convince Santa himself. She whimpered, poking at the fluffy diaper around her waist, a thousand thoughts washing through her mind at once. What was she going to do? How was she going to get out of this?! She didn't want to be a baby!

"Oh, there you are!" She flinched as she heard an unfamiliar voice, looking up to see an attendant standing over her. "Are you ready to start, sweetie?"

"S-Start?" Matti furrowed her brow. "I-I don't..."

"You'll be great!" the woman smiled, giving her a hug. "You're so adorable, nobody will care if you mess up!"

"No, I'm not!" Matti sulked.

"Don't be silly," the attendant chuckled, taking out her phone and snapping a picture. "Look at you!"

Matti gasped at the sight of the photo, feeling her diaper warm and swell beneath her. There was no trace of her adult self in it, no sign that she'd ever been anything more than this. Even the frames of her glasses had turned pink, to match the rest of her outfit in betraying her by shrouding her in her least favorite color, her red hair pulled into a pair of tiny ponytails framing her chubby-cheeked face.

"I-I hafta see Santa," she whined.

"Santa's not going to be ready until after you perform," the woman told her, easily lifting her up, setting her down on the floor. "So you'd better get started."

Over the speakers, Matti could hear the music change to the Dance of the Sugar Plum Fairies. She shook her head, turning back towards the attendant, only to be pushed forward with a pat on her padded bottom. That propelled her forward a step or two, and, before she could stop herself, she felt her feet begin to move on their own, her body taking over, starting to dance.

She had no control over it, no way to stop it, to keep herself from twirling down the aisle, tutu bouncing, showing flashes of her soggy diaper as she helplessly moved closer and closer to her family at the far end of the car, her heart beating faster and faster as she went, even before she started to look around.

Was that Erik, sitting there, beside his little sister, staring? As soon as she caught his eye, for just a split second, she knew it was, and that he recognized her, despite her diminutive form. Did he even live near her? She couldn't remember for sure, though she knew that if he told the rest of their friends about this, she'd never live it down.

And, speaking of that, seeing him there was nowhere near as bad as who she saw next, the series of cheerleaders from Kelsey's squad sitting there, watching and texting back and forth, smirking at the sight of her. She couldn't stop, however, no matter how much she wanted to, how anxious it was making her feel as she saw them snapping pictures, her tummy grumbling nervously.

Or was it nerves? She had plenty of them to go around, knowing how close she was getting to her family, and how unlikely it would be for them not to recognize her, when everyone else seemed to have. And yet, the longer she went, the more it started to feel like something else, until, as the song reached its crescendo, she arrived.

All at once, her body halted, its memory of ballet seeming to return to normal, leaving her standing right in front of them in her tutu and diaper, blushing. "Go on," her mother urged sweetly. "You can do it!"

Matti fidgeted, wanting to comply, to keep going, knowing that the more she moved, the harder it would be for them to see it was her, though, from the smile on her sister's face, she suspected she was already too late. "Yeah," Kelsey smirked. "Go ahead. Show us what you got."

That was when the cramp hit, her tiny body no match for it. Instantly, she felt her knees bend, her tummy push, her diaper swell beneath her leotard as she gave them all the grand finale to her humiliating dance by thoroughly, helplessly, filling her pants.

"You poor little thing," Kelsey scooped her up, setting her down on her lap, giggling as Matti's nose wrinkled. "What are we going to do with you?"

"All right, everyone!" Matti could hear the attendant announce. "Santa is waiting in the next car!"

Matti whimpered, trying to squirm free of her sister's grip, to run up to Santa as quickly as she could, but she was no match for her. She had no idea why Kelsey had a pacifier in her purse, but she did, sliding it into her mouth, Matti feeling herself start to suckle on it automatically, unable to fight her instincts.

"I'm sure Santa is too busy to deal with stinky little girls like you," Kelsey grinned. "You're staying right here." Matti shook her head, trying in vain to plead with her sister around her paci. If she couldn't talk to Santa, she'd be stuck like this until at least next year! She couldn't let that happen! She was supposed to graduate next year, along with Kelsey, but that couldn't happen if she was spending her days stuck in daycare, struggling with her potty training! There would be no doubt which of the two of them was more mature now, or how big of a baby Matti really was.

"I do," Santa told her with a wink.

Matti blinked, shaking her head, blushing even deeper as she snapped out of her fantasy, finding herself still standing outside the bathrooms, talking to Santa. "O-Oh," she stammered, shaking her head, trying to clear away the images she'd been imagining. "C-Cool."

"Yes, it is," he said. "Be sure to come see me!"

He turned and walked away, leaving her to duck into the bathroom for a minute to try to calm down, washing her face, looking at herself in the mirror to confirm that she was still the same age she ought to be.

There were, of course, none of her college friends or acquaintances there, not that she saw as she shuffled back to her seat, another reminder that none of that had been real, thankfully. "What took you so long?" Kelsey asked snottily.

"N-Nothing," Matti shook her head.

She was still blushing about it by the time the food came out, however, and as the carolers sang to them all, and especially when it was announced that Santa was ready in the next car. "Are you going to go see him?" Kelsey teased. "Are you gonna sit on his lap and tell him what you want for Christmas this year?"

Matti squirmed, thinking about how, in her mind, she'd seen herself sitting on Kelsey's lap, imagining being like that, a ballet-obsessed little girl in a messy diaper, and going to meet Santa. "O-Of course not," she swallowed, staring out the window. "Th-That's for the little kids."

"Nonsense!" Matti jumped, turning to see the attendant she'd seen in her dream standing by their table. Matti must have seen her earlier, she reasoned, even if she didn't consciously recall it, even if she had no memory of seeing her except in her mind. "Santa has something for all of you!"

She wouldn't take no for an answer, standing by until they got out of their seats and joined the line. Matti's whole face felt like it was on fire, the heat growing the closer they got to Santa, her only comfort the fact that Kelsey didn't look much happier.

"Ah, Kelsey!" he smiled when he saw them, reaching into his bag and taking out a small box. "This is for you!"

Kelsey looked around, confused at how he knew her name. "Umm... Thanks," she said uncertainly.

"And Mathilda!" he continued. "I have something very special for you!"

Matti breathed in sharply as she watched Santa dig into his bag. Was she about to have to open a package of diapers in front of her family?! Those had definitely been on her list, too, and, while they might be better than her regressing for real, it wasn't by much.

"Y-You shouldn't have," she managed a chuckle.

"Of course I did!" Santa winked. "You've been a very good girl this year!"

She cringed as he pulled his hand out of the bag, not knowing what to expect, but it was another small box, exactly the same size as Kelsey's. "Merry Christmas!"

The same thing was inside both boxes, they found when they opened them, an ornament of the train, the year stamped onto the side. Matti was relieved, of course, though also a little disappointed.

At least, until Christmas morning. Matti did her best to contain her excitement, not wanting to get the rest of the family up too early, since she knew she was too old for it to be cute anymore to wake them up at four in the morning to open presents. She rolled over several times, cuddling back up under her covers, before, finally, letting herself get up and stumbled across her room to the light switch, nearly tripping over something in the middle of her floor.

With the lights on, she could see a large box sitting there, perfectly wrapped, with a tag on it claiming it was from Santa. She tilted her head, looking over at her bedroom door, which was still closed, the way she'd left it when she went to sleep. Surely she would have heard somebody come in through there, wouldn't she? There was a squeak in the hinges that she always told herself she was going to fix the next time she was at home, but never did.

Of course, she knew it was from her parents. Was it there to appease her for a few minutes, to give her something to open on her own so she wouldn't be tempted to get everybody else up as early? It was possible, and that did the opposite, made her want to wait and open it with everything else to prove to them that she had some self-control.

The writing on the tag didn't look like her mom's, however, or her dad's, or even Kelsey's. She stared at it for a few moments, then, unable to help herself, she tore into it.

She was glad she did, too, grateful she hadn't opened it up in front of her family. It was a package of diapers, big and fluffy, and covered in a dinosaur print. She wanted very much to rip them open, take one out, but if she did that, she wasn't sure she could resist putting it on, and she couldn't do that, not yet. That could wait until tonight.

She smiled, already impatient for the day to be over, despite all the other presents she had to open under the tree, the thought of diapering herself under her PJs, of crawling into bed like that, the very best way she could imagine to end her Christmas.

She was so excited, in fact, that she didn't think about where they could have come from, what that meant. Who could know about her secret, to be able to give those to her, and get into her room?! She didn't even notice the other part of her gift until after she had carried the package to her closet and tucked it into the corner, turning back to see what had fallen off and fluttered to the floor. She bent over, seeing that it was clearly a photograph, picking it up and turning it over.

She blushed, sitting down at the edge of her bed in shock. There she was, staring back up at herself, sitting on the train in her ballet outfit, her wet diaper peeking out from beneath her leotard and tutu, regressed into a toddler.

It was the picture the attendant had taken! But that had only happened in her imagination. Hadn't it? She squirmed, looking down at the magic of Christmas, right there in her hand. It looked like she had an answer as to who had given her the diapers after all, as ridiculous as it seemed, and as much as she knew Kelsey would tease her if she ever mentioned it to her. Apparently, Santa had gotten her letter, and, despite some of the very naughty things she'd done that year, she was still a good enough girl for him to answer it, and give her exactly what she deserved.