Up until encountering gnomes, manotaurs, and surviving a literal apocalypse, the existence of creatures like Santa, The Easter Bunny, and Jack Frost had been hard for Dipper to believe in, and in a way, still were at times. Adults didn't believe in them, after all.

Mabel had no such issues, having instead taken the weirdness of Gravity Falls as proof of their existence. She may not have advertised it – the twins had enough issues in school without adding teasing about believing in that particular set of paranormal creatures to their ammunition – but she believed anyway.

She just hoped they didn't turn out to be a disappointment, like the unicorns.

But if the rest of it, everything else Gravity Falls had thrown at them, could exist, why not Santa, or Jack Frost, or any of the others?

Which was the reason why the twins were stomping through the snowy woods of Gravity Falls.

Despite knowing they'd left the town fine after the whole...unpleasantness, as the citizens referred to it (complete with pause), they'd wanted to come back and see for themselves. And summer break was too far away, even with the hand-held portal that Grunkle Ford had given them, the Skype calls and text messaging with Soos and Candy and Grenda and Wendy. They wanted to see everyone again in person.

So they were up here for winter break. Part of it, at least. Mabel loved all the holiday, so they'd need to be home for Christmas, and possibly Hannukah too, though she was making noises about getting the Grunkles involved.

But that was for later.

For now, Mabel had heard rumors online about Jack Frost, and she wanted to see him. So both kids were out in the snow, tramping through the woods.

Dipper wanted to be as amazing at investigating as his Grunkles, but even with Mabel's knitting the cold was really starting to get to the (technically) teenager. Which meant his focus wasn't as on the hunt as he wanted it to be.

"Dipper! I saw something!" Mabel gasped, breaking Dipper out of his thoughts.

Up ahead, Dipper saw the flash of blue Mabel was pointing to and perked up, dashing forward to join his sister as they darted as best they could through the thick snow, following the flashes of blue and brown they could only just see, blurring between trees and snow and moving almost too fast to follow.

The strangest part was how it kept pausing, as if making sure they were following, while never staying still long enough for them to get a good look at it.

Suddenly they broke out of the trees, skidding to a halt. "Aw man, we're back at the Shack," Dipper groaned. "It lead us to the Shack. Why would it do that?"

"Maybe it's for the best," Mabel said. "I'm freezing, and I hear hot chocolate calling our names, bro! Hot chocolate with edible glitter!" She set off towards the Shack with Dipper close behind, laughingly protesting the glittery addition.

Dipper was up late that night after Mabel was asleep. Finding Jack Frost had been her idea, but now he was just as invested – and he wanted his sister to see that spirit.

Bill may have destroyed the journals, but they'd reappeared later, just as the Shack had, as everything else Bill had destroyed had. Mostly. There were some scorch marks, but they were legible and real. Grunkle Ford was still already working on new ones, better ones, ones that weren't tainted with paranoia and demon induced hallucinations.

Still, Ford and Dipper had scanned through the old journals for anything that might have been helpful, whatever little there may have been, while Ford told the twins everything he'd ever heard about Jack Frost, but it wasn't much. Even as into the paranormal as Grunkle Ford was, some spirits just had so little information out there that it was difficult to prove anything concrete.

Well, Dipper wasn't going to let a little thing like lack of empirical evidence get in his way! Outside of Gravity Falls these things might have still been just myths, but they were proven fact here.

Chewing on his pen, Dipper bent over his new journal, the one he'd created for his own research, emblazoned with a pine tree on the cover (Dipper had made Mabel one to match, with her shooting star on the front, and though she used it more like a scrapbook, she still had quite a few notes in there that even Dipper and Ford had missed. Dipper had to admit that his sister was smarter than people gave her credit for) re-reading everything he'd gotten written down so far. It wasn't much, but he'd done more speculation with less information than this.

Dipper woke with a start, bent in half over his book. He jerked upright, pen clattering out of his loose grasp to the floor.

The window was frosted over in intricate patterns. For a moment Dipper stared at them, his sleep fogged mind unable to process at first just what was wrong with the image in front of him.

It was winter. It snowed here in Oregon. There should be frost on the window.

Then the wrongness of the image before him snapped into focus.

Frost was supposed to look vaguely like ferns, wasn't it? Not like a big eyed kitten in a boot!

Scrabbling, Dipper grabbed for the camera on the nightstand, the one Mabel had left there, one of the many Mabel had started to leave about after the whole Bill incident.

He snapped a picture of the window and shivered. It was like an icebox in here, what gave? Wasn't heat supposed to rise? It had been fine, if a bit chilly, earlier, what with the Shack's poor insulation...

Something rattled on the other side of the room.

Dipper spun, the light from his flashlight flickering across the familiar attic.

There was a flash of brown and blue at the corner of his eye, a trail of frost and ice inside the house where it didn't belong.

Eyes never leaving the trail, Dipper slipped out of bed and poked his sister.

"Mabel! Mabel, wake up!" he hissed.

"No. Never. Lemmie sleep forever," she grumbled, rolling over.

"I think Jack Frost got in!" Dipper hissed. "Come on, we gotta hurry before the trail melts!"

Mabel shot up in bed, eyes wide and all traces of sleep gone.

The twins followed the ice trail cautiously into the deeper parts of the attic, where actual storage took place. The ice was melting quickly in the Shack's warmth, which was returning by degrees, so they didn't have much time before it would be gone.

Then they turned a corner and were temporarily blinded by a flurry of snowflakes.

When their vision cleared, two jaws dropped and two sets of eyes opened wide at the sight of the teenager in their attic.

It had to be Jack Frost, not a human teen – no human teen would be that pale, or be barefoot in this weather, or have frost and ice coating their clothing. (The hair, white as snow, didn't count. Gideon's hair was still pure white, and he was ten.)

He had some kind of staff, and was currently using it to poke curiously at a carpet that had fallen over at some point and half unrolled onto the floor.

His head flew up when the light from Dipper's flashlight hit him, and he stared at the kids like a deer in the forest.

The three stared at each other, a frozen tableau, until the spirit broke it. Still unmoving, his eyes lit with a tiny spark of something that looked like hope, hope that had only recently started to be rewarded, too recently to take away the sting of fear that it would, once again, be disappointed but needing to believe despite that, Jack Frost whispered, "Can you...can you see me?"

"See you? We've been looking for you all day!" Dipper cried, stumbling closer as his sister began to vibrate with glee.

Jack's eyes went wide, and he looked to be as thrilled and excited as Mabel. "Really?" he said in disbelief, his feet shifting on the carpet and kicking up tiny sparks, unnoticed by all three.

Dipper flipped open his journal, which he had grabbed alongside his flashlight, and yanked out the pen kept inside, clicking it rapidly. "Yes, really! You're real? Oh, man, I have so many questions!"

Jack laughed as Dipper started pacing, kicking up sparks each time he stepped on the carpet. Mabel looked up at the spirit with wide, sparkling eyes, a high pitched squeal barely contained behind clasped hands. "Well, I can't promise anything, but I might be able to answer a few..."

"What about the others?" Mabel burst out, unable to contain it anymore. "Santa and the Tooth Fairy and..."

"Real, real, they're all real! Every one of us is so real," Jack said quickly, laughing as Mabel grabbed his hands and spun them in a circle, tripping and falling back onto the carpet.

At the same moment, Dipper stepped onto the carpet, his foot hitting the bright weave at the same moment that Jack landed on it.

There was a bright flash, one that sent everyone to the floor. As the speckles faded from her eyes, Mabel finally saw the pattern of the carpet in the beam of Dipper's fallen flashlight and let loose a few of the words Grunkle Stan thought she hadn't heard him use.

Jack Frost and Dipper sprawled like the teenagers they were over the body-swap carpet, momentarily stunned.

Dipper, now in Jack's body, recovered first, sitting up and rubbing his head. "Oh, man, what..." he began to say, before getting a look at his hand. "Not again! I thought Stan destroyed this thing!"

Jack, for his part, was staring down at Dipper's body blankly. He edged himself up slowly before raising trembling hands, looking at them in shock.

"Hey, it's gonna be okay," Mabel called, holding out her hands. "It's a body swpping carpet, you just gotta get another charge going and you'll switch right back. Like bwooomp," she added, giving sound effects to her hand motions.

"I...wow," Jack laughed, wrapping arms around his borrowed body. "I...I forgot what it felt like to be alive. Wow. I...that sounds super creepy."

He looked over at Dipper in his body and grinned. The gamine grin of pure mischief looked out of place on Dipper's face.

"Wow, is that what I really look like? It's been ages since I saw a mirror. It's easy to switch back, you said?" Jack asked. Dipper and Mabel nodded slowly, watching Jack as he scrambled to his feet, still grinning. "Come on, then – let's have some fun first!"

Dipper flopped down on the ground next to his sister, exhausted. Mabel flopped over his stomach – technically Jack's stomach – just as worn out as her brother.

"Broseph, I didn't think your body had that much energy," she moaned. "Well, I mean, you must 'cuz of all those times you stay up for like a week at a time but..."

"Uuugh," Dipper groaned. "This is ridiculous."

Dipper blinked and groaned again as his own face suddenly appeared over him and stared at him upside down. "Doesn't that weird you out?" he asked the spirit currently inhabiting his body. "It does me."

"Mmm, not really," Jack said with a grin, tilting his head to the side. "Come on, you let me play, now it's your turn. Want to play with ice powers?"

The twins grinned at each other, perking up. "Do we!"

Three hours later, and they were looking at a massive creation of ice and frost, a playground that only the Guardian of Fun and two thirteen year olds could conceive of.

"That's...not going to melt anytime soon, is it," Dipper said, even as Mabel whooped and yelled, dashing and sliding her way through the structure. It was only as sturdy as it was because of Jack's help, though with Dipper's math skills they'd made it bigger and more elaborate than Jack had originally thought it could be.

"Nope. Wish we would work together more often," Jack said, elbowing Dipper in the side. Dipper, still awkward in the unfamiliar and much taller body, stumbled. "But eh, you two live in California, right? I don't get to play much there."

"We're planning to spend more time over our winter breaks up here at the Falls," Dipper offered.

Jack grinned. "Great! Let's switch back, so I can show you how to have some real fun with this monster and later we can start planning out some more stuff."

"Ooh! Ooh! My turn!" Mabel called from inside the ice palace.

"Once we swap back, I'm taking you two flying," Jack called back.

"WAY BETTER!" Mabel yodeled as she flew down the huge slide spiraling around their creation.

Dipper thundered up the stairs after Jack in his body, gladder than ever that he'd listened to his sister and believed.