So I wrote this a long time ago in a frenzy and just remembered it :) I love the idea of little Elsa and I've always loved the idea that Jack was a big brother. I had to combine! Please enjoy cross-over oneshot origin story:

Elsa examined her mug closely. She looked all around it and under it, and finally above. It looked frozen but, she glared at the cup, that wasn't what she had tried to do. She tapped the top and gasped with delight. It made a swishy-echoy sound. She pressed her finger harder on the ice and with a soundless crack the layer of ice broke in the middle with the edges thick enough to remain frozen. She smirked. She had done it.

Elsa ran to the window and looked out the panes, careful not to touch the edge, lest her powers freeze the window again. There, just there in the distance, she could see the big pond at the edge of the forest that had yet to freeze over for the winter. She lifted her chin to get a better look and smiled. That would be the best place to try it for real. But, her face suddenly fell and she ducked her head. Should she be having this much fun with her powers? They were so dangerous. She peeked a look outside again without moving her head, but it was at the edge of the forest. No one ever went over there.

She took deep breaths and started to dance on the spot. Her parents would be at meetings in the throne room till Dinner and Anna was somewhere in the castle probably talking to the paintings. This was perfect! She clapped her hands and ran to the window again. Carefully, ever so carefully, she opened the panes. Looking behind her constantly, she slid out onto the roof. She was smiling more than the winter sun above as she shut the window with just enough of a crack to open it again. And with that, she slid down the roof of the castle, her powers gathering a pile of pillow-like snow at the bottom to catch her. She giggled at the tickle bumps and squeaked, coming into a puff at the end. She hadn't done that before! Sometimes her powers were such fun! She suddenly gasped and put her little hands over her mouth. Had anyone heard her? She stood frozen to the spot for who knew how long before she ran quickly to the forest. Her room faced the mountain side and the village was way on the other. No one saw the little ice princess.

Finally at the pond, gasping for the freezing air, she stopped and looked at the still, almost glass-like surface of the blue gray bowl of water in front of her. She narrowed her eyes and smiled. Alright, it was time to test herself. Extending her arms and waving them slightly like she had done for her cup, a thin sheet of ice began to form and extend to the center, turning the pond white and blue. Her tongue stuck out in concentration, Elsa squinted at the surface of the pond as it all came together. The center closed up and she stopped her powers.

She smiled and gazed in awe at her creation before her. A smooth mirror surface glinted and winked in the bright sun. Elsa looked down and slowly, ever so slowly, eased herself onto the edge. It held. She skidded closer and closer to the middle. A small, ever so small- a whisper of a click reached her sensitive ears and she stopped. Yes. This is exactly what she had wanted! She had been trying so long to freeze something and leave the rest unfrozen. She couldn't stop smiling, she was learning how to control her powers! Skidding back as fast and safe as she could she admired her masterpiece for a little while longer.

"Jack!"

Elsa jumped and whirled around.

"Jack, wait up! Mom said to be careful!"

The cheery laugh that followed told her it was a pair of siblings coming up the mountain.

Elsa panicked and turned right around and ran. Her running and panicking soon turned into skating and sliding down one of her emotion- made slides right back to the castle.

Back safe in her room, she sat on her bed and thought about what she had just done. She smiled and fell down on her back. Yes. She did it. She had run and made something beautiful. Something dangerous. Something wonderful. Elsa made a few large snowflakes with her hands. Beautiful. Powerful. Dangerous. Cold. Elsa smiled. Sometimes, not often, but sometimes she loved her powers.

That same night, Jack Frost was pulled from a pond by the moon.

A/N: Please put your nose to the screen so I can boop it to say: Thank you SO much for reading this crossover! It was super fun to create and I love these characters so much, have a great rest of your day, night, evening, morning, twilight, fortnight, wherever you are in the galaxy, have a good one :D