Disclaimer: Adventure Time belongs to Pendleton Ward/Cartoon Network.

A/N: This is really just something I wrote to amuse myself, ever since I saw 'Earth and Water' I kept wondering to myself what would have happened if Flame King had just let Princess Bubblegum find a way to contain Flame Princess' power. It grew from there into "what if PB adopted her instead?" so this is the AU that resulted. I will possibly write more, but if I do it'll be a series of vignettes rather than a proper story. Also, baby Flame Princess is the most adorable thing that has ever appeared on the show.

Phlogiston Theory

"Find some way to contain her power, or I will." Princess Bubblegum glared at the Flame King as she spoke. She'd never have thought that she'd have missed the old King, but at least he had never abandoned his children on the surface. Not without an army at their backs anyway.

"Pssh, fine," Flame King replied dismissively. "You do it then. But not here."

Bubblegum frowned, glancing down at the baby in her arms, who gurgled at her happily, completely oblivious to the situation. "But she's your daughter," she said uncertainly. "You want me to just take her away? You'd LET me just take her away?"

"Yeah, whatever." The Flame King gave a dismissive flick of one wrist. "Do what you like with her, science junk, whatever. But be warned, Gum Princess…" He leaned down from his throne, his flames blazing to at least twice their usual height. "If you douse her, there will be consequences. Dire consequences."

"I'm not going to douse her!" Bubblegum replied indignantly. "I'm going to…" She broke off in mid-sentence as she realised that Flame King was laughing at her.

"Nah, just kidding," he chuckled. "No biggie if you douse her. Would actually suit me better come to think of it."

Princess Bubblegum was many things, ruler, scientist, diplomat, cold and ruthless logic-driven automaton if Marceline had anything to say about it, but nobody could have accused her of not caring for her subjects. And for Flame King to show such callous disregard not just to one of his subjects, but his own offspring, was a concept that was completely alien to her.

Hearing her father's laughter, the little flame elemental in Bubblegum's arms started to chuckle, her body glowing as she grew hotter. The princess was sure that the suit would be able to withstand the heat, it was made for travelling in the Fire Kingdom after all, but the arm the little girl was cradled in was getting uncomfortably warm so she switched her over to the other. Looking back at Flame King, she could see that he was grinning at her malevolently.

"Not going to be as easy as you thought, huh?" he asked, his grin widening at the look on her face. "Okay, how about you just drop her off in the woods to perish and we pretend this whole thing never happened, deal?"

"No deal," Bubblegum snapped. "I'm taking her back to the Candy Kingdom."

"Your funeral," Flame King shrugged. "Hope you like cremations over there. Oh, and if she burns it to the ground that means technically the Fire Kingdom's conquered it, so it becomes mine. It's the law. Are you even listening?"

Princess Bubblegum was, in fact, not listening at all as she turned her back on her fellow ruler and headed off back along the path that led to the surface. "Stupid Flame King…" she muttered under her breath, then glanced down at the tiny fire elemental she was carrying. The little princess was still looking up at her, but now she was also doing her level best to fit her foot into her mouth. Despite herself, Bubblegum smiled. "Well, I guess I'm going to have fun explaining this to Pepbut, huh little one?"

Flame Princess' only response was a muffled gurgle.

o.o.o.o.o

"Welcome back milady, how did the trip to the Fire Kingdom..." Peppermint Butler's voice trailed off as he saw that the princess was still carrying the infant fire elemental and looked distinctly displeased. "Not well then?" he asked, taking the protective suit's helmet from Bubblegum as she removed it.

"Well, Flame King is a butt, I knew that already," the princess said with a scowl. "But I didn't know just how big a butt he could be until now."

"Did he refuse to see you?" Peppermint Butler hurried along behind the princess as she headed upstairs to her laboratory. "He didn't care that you found his lost daughter?"

"She wasn't lost," Bubblegum replied, her expression softening slightly as she looked down at the baby princess who was cooing and chortling as if this was all some wonderful adventure for her. "He had her left on the surface on purpose."

The small mint man frowned. "If that's his idea of an invasion, he's certainly not the same tactician his brother was."

"It wasn't." Stepping into her lab, Bubblegum carried the little fire girl over to one of the fireproof work surfaces and gently set her down on it. "He just wanted rid of her because he thinks she'll usurp him."

"Well, he does rule the Fire Kingdom," Peppermint Butler shrugged. "That is a hazard of the job. So, what are you going to do with her?"

Rummaging in a drawer full of writing and drawing implements, Bubblegum pulled out a stick of artist's charcoal and carefully handed it to the baby princess. The little girl gurgled happily and started to gnaw on it toothlessly, like any candy child would have done with a stick of barley sugar. "I'm keeping her," she replied, as if it was obvious.

"But milady," Peppermint Butler protested. "She's a fire elemental! She's dangerous, especially since she's so small. She could set the entire castle ablaze. Wouldn't it be better to send her to the Rock Kingdom or the Gem Kingdom? Maybe even the Nightosphere, I'm sure Lord Abadeer..."

"It's not up for discussion Pepbut." Bubblegum's tone made it clear that she'd made up her mind already. "She's my responsibility now so I can't just pass her off to someone else. Besides," she added, leaning her elbows on the bench and studying the tiny princess with clinical detachment. "I'm sure I can find a way for her to control her powers."

Peppermint Butler wasn't so sure. The princess generally had good intentions, but those did pave the road to the Nightosphere, along with a significant portion of that dimension's civic spaces. He hadn't been around three centuries ago when Bubblegum had tried to isolate the compound that caused candy people to explode, but he'd seen the result on occasion and still jumped when anyone said the word 'unacceptable'. Someone else had been there though, and while dropping her name was risky, it was a chance he was willing to take if it meant not waking up on fire.

"Well..." he began hesitantly. "Maybe you should ask Ms Abadeer what she thinks?" Old Mr Creampuff had told him to be very, very careful when using that name, but also that it guaranteed results. He hadn't been wrong, Bubblegum's head immediately snapped towards him, but her glower indicated that this might not have been the best idea after all.

"Marceline?" the princess snapped, eyes narrowing. "Well, maybe if she hadn't upped and left one hundred and eighty-five years, three months and fourteen days ago I would ask her. But since I haven't even had so much as a letter from her, I hardly think she cares!" Her voice had risen to an angry shout, and the little fire elemental began to whimper at the noise. Bubblegum felt a rush of dry heat wash across her as the tiny girl's elemental matrix shifted in response to her emotions.

Picking the baby princess up again, Bubblegum rocked her soothingly. "Shhhhhh, it's okay, I'm not mad with you." She shot a sharp glance towards Peppermint Butler that clearly told him it had been a mistake to mention Marceline. "She's staying. End of discussion. I want you to go and fireproof a room for her like we'd do for any other visitors from the Fire Kingdom."

Groaning inwardly, the mint man bowed. "As you command milady." The little elemental had calmed and was now lying quietly in the candy princess' arms, gurgling as Bubblegum smiled at her. This time, Peppermint Butler's internal groan was heartfelt. He had never thought that the princess could possibly have a biological clock given her age and apparent immortality, but maybe he'd been wrong. "You're not getting attached, are you Peebles?"

"Pssh." The noise Bubblegum made was just as dismissive as the similar one the Flame King had made earlier. "Don't be silly Pepbut, this is for science."