Disclaimer: I don't own anything. A.N. For Sherlock challenges' March prompt, "experiment gone wrong", and my dear Sendai, who read my latest parentlock and wanted more of it (also as an apology in advance because her birthday fic's chapter this month will be rather bleak).

Like papa, like daughter

Rosie is a curious, adventurous child. Given who her parents are, it's no surprise. It is also no surprise that her affectionately dubbed 'mad scientist' of a papa – despite now being confined to 221C for his more questionable activities – would want to involve her in scientific experiments from an early age.

Not as the guinea pig, John has been rather vocal on the matter, and Sherlock agrees. After all, needing a test subject means you're not sure of the results in the first place. While the sleuth knows his beloved is strong enough to endure whatever random trial might be needed to prove an alibi or solve a case, he is as horrified as John by the mere thought of the chance of hurting their child (yes, he's claiming her) in any way.

It doesn't mean that Rosie cannot participate in some science experiments – as an observer, and, if the situation allows it, even lead scientist. Sherlock has even gone to the trouble of researching age-appropriate scientific endeavours. Sure, he could have just asked his mummy – they bonded rather deeply observing the combustion of various objects – but frankly, he mistrusts her parenting skills. Look how he himself turned out, after all (and he's not even judging his siblings).

Fine, he might have lowered the suggested age threshold a tiny bit. But Rosie – as expected from John's child, truly – is exceptionally bright, so anticipating experiments by a year or two really should not be a problem.

As much as this is their special time – Sherlock's and Rosie's – John, if he's home (which does not always happen) likes to be present. He can't help but grin at both his loves' passionate and deeply serious endeavours. He's always had a soft spot for Sherlock's experiments, even the more apparently harebrained and unsanitary ones, no matter how much he grumbled.

These, which do not even require human body parts? (Yet, at least…in a decade or so, he suspects he will again be surprised by the contents of their fridge.) They are, frankly speaking, adorable. And Rosie loves his heartfelt 'amazing', and 'brilliant, love', which she gets for showing off what she's learned, just as much as her papa always did.

Of course, it would have been too much to ask that Rosie would acquire from her papa – idolising little sponge that she sometimes seems – only the best traits, encyclopaedic knowledge and scientific interests. She's unprecedented in the history of children (actually, that was one of Sherlock's tweets about her during the christening, which John loves to quote), but she's still human. Which is why they find themselves in situations like these (thankfully, not very often).

It's Sunday, and Lestrade comes over with what he swears is at least a nine. The consulting detective suspects no more than a 7.5, at best, given Giles' tendency to overstate things, but if he's too picky, the inspector might decide to retaliate by excluding him when he's gagging for a case. So he accepts it, with a put upon sigh, and after a moment of uncertainty on his blogger's part, Mrs. Hudson assures them that she's perfectly capable of taking care of Rosie, thank you very much, and she likes her Nana, does she not?

At the child's enthusiastic nodding, they leave, thanking their landlady profusely, while she waves them away. It really isn't a good idea to let the sleuth go alone, what with him not understanding the concept of backup unless John's the one providing it.

Once at the crime scene, the detective is outraged. True, the murder method is, frankly speaking, bizarre – a reason that John finds the crime interesting and will probably mention it on the blog. But as far as actual mystery goes, this is barely a 6+. Honestly, if the so-called forensics and police officers would just observe what is in front of their noses, they would have solved it in twenty minutes, since it takes Sherlock two to figure it out.

Since they're already out, they might as well take the extra hour to trap the murderer (the victim's half-brother) and deliver him to the Yard. Rosie will pout if they interrupt whatever game Mrs. Hudson has undoubtedly set up for her.

When they get back to Baker Street – seriously, the murderer was disappointing all around – their heart stop in unison. There's a smell of smoke, and the soft sound of Rosie crying. What happened in their absence?

John slams open the door to Mrs. Hudson's flat, yelling for her and his daughter. Rosie beelines for him and hugs his legs tight, sobbing and sniffling. Among the sobs, she wails, "Nana hates me now!"

Sherlock goes to explore, and finds their landlady putting out a singed curtain. "Just you, young man! What are you teaching to that child?" she asks, in a huff.

"Science, Mrs. Hudson," the sleuth replies, in his most haughty tone.

The old lady deflates immediately, "Science is not to be sneaked on people! I went to the bathroom for three minutes and came back to this," she explains, with a wide, defeated gesture encompassing the scene. She's clearly a bit shaken.

Sherlock nods sternly and goes back to his lover and child. With all the data, he now knows what to do. John is trying to comfort Rosie, vaguely assuring her that of course Nana loves her, she always will, but that doesn't agree with what the child has just gone through, so of course it has little effect.

The detective goes down to his knees, to be level with Rosie, and asks quietly, "What experiment did you try during Mrs. Hudson's absence?"

Between sniffles, she replies, "We were having a tea party, so I did the teabag rocket one. Twice, because you told me science is all about checking your results with repetition. The first one was good, but the second…"

"The second burning, flying teabag caught against the curtains, which, being flammable, went happily up in flames?" the sleuth finishes for her when her voice is caught, overcome by sobs once again.

Rosie nods vehemently. John doesn't say a word, but hugs her tighter – thinking of his precious daughter playing with fire unsupervised is scary – and raises silently an eyebrow at his lover, clearly questioning the wisdom of such an experiment in the first place.

"This is a lesson all scientists have to learn someday, and better do it soon – control your variables. You thought that a kitchen was equal to any other kitchen. But there's a reason our kitchen does not have curtains at all. I've burned through one too many sets of them," Sherlock explains, with a little, conspiratorial smile.

"You did, papa?" Rosie asks, sobs finally waning in the wake of shock.

"Absolutely!" the man confirms, smiling still.

John adds, "It's a long time since we had to – your papa finally learned, about the time you were born, because he didn't want you to be at risk – but we used to call the fire brigade way too often. It wasn't just curtains which went destroyed. I suspect that if I bring you along to the fire station, they'll treat us as old friends."

The child's mouth opens in a silent, big, wondering, "Oh!"

"Why did you think Nana hated you?" Sherlock presses on.

"She yelled at me," she mumbles, looking down.

"And that's your data. But you have to consider all kind of explanations before setting on one. For example, would you yell if I was concentrating on an experiment and risked hurting myself? Say, if I mistook one of the cups of tea Daddy keeps providing for a cup of chemicals?" The consulting detective asks.

Rosie actually thinks about it, frowning in concentration, before deciding, "Yes?" sounding unsure that'd be proper.

"Of course you would, love. Because with an airhead like him, there's a need of something loud to bring him back to earth," John adds, shaking his head at his two mad scientists.

"And because you'd be scared for me, and didn't want me to get hurt," Sherlock remarks, eliciting vehement nodding from his daughter.

"So… despite all the burnt curtains, and tables, and just lots of burning, and accidental poisoning, and indoor shooting, and all the bit not good things I really shouldn't mention, because this is not about teaching Rosie that, do you still love me, John? Or do you hate me?" the detective asks, finally getting up.

"Don't be ridiculous, papa!" Rosie admonishes, giggling. Her parents are preposterously, almost disgustingly in love with each other, and anyone with half a sense can see that.

John grins broadly. "Yep, don't be ridiculous, love. Or I might really have to kiss you, and we have a misunderstanding to clear up now."

"Mrs. Hudson! If you're quite done with that, we need you!" the sleuth hollers, after grinning blindingly back to his partner.

The old lady joins them immediately, secretly worried. Such a request from Sherlock can mean anything from a request for tea to an acid spillage. Not that there should be any acids in her hall, but still.

As soon as she shows, the sleuth queries, "You do remember all the damages I've caused to the flat in my pursuit of science and various attempts to be entertained, don't you?"

"I might be old, but I'm not yet demented, young man. Of course I remember all your so called accidental bouts of destruction," Mrs. Hudson snaps rather sternly.

"And do you hate me?" the consulting detective retorts, ignoring her insinuation that he meant to cause havoc – most often, upset because of some hurdle in his relationship (or lack of one) with his blogger.

"Of course I don't, Sherlock. If you have to ask, this might be more of a matter for John – you're not feeling well at all, and possibly brain damaged," the landlady quips, shaking her head.

The detective grins at her. "Don't worry about my health, I'm perfectly fine. Just gathering data to make a point. Since you've forgiven me that much, is it correct to say that you don't hate Rosie for one measly curtain, either?"

Mrs. Hudson gapes in shock. "What? Of course I don't hate Rosie! Where did you get that idea, dearie? I thought you were crying because you were afraid of the fire," she replies, opening her arms for the child to run into and hugging her tight.

"She's Sherlock's daughter, Mrs. Hudson. No mere flame is likely to terrify her," John points out, not sure if he's praising their baby girl or not. The sleuth flushes brilliantly at that. Of course, he's been acting like Rosie's parent all along. Still, with her genes not matching his (which is for the best – what with his weakness for addictive substances and general flaws), hearing that stated aloud wakes up all sorts of butterflies in his stomach.

"I'm sorry if I scared you, Nana," Rosie mumbles in the old woman's embrace.

"I just don't want you to get hurt, Rosie, love," Mrs. Hudson points out kindly, "next time, let me be there as lab assistant, please?"

The child nods solemnly.

Later on, in their own flat, when Rosie runs to get Bee (years might have gone past, but it will take a lot longer to get rid of Bee, especially when she's upset) John whispers, "I feel like we should celebrate her first experiment-related destruction…but I'm afraid that would send the wrong message."

"You've never celebrated mine," Sherlock replies, a hint of pout already forming.

"True. We'll pass on that, then," his blogger agrees, kissing it away.