Mayuri didn't know why everyone thought watching Yachiru was so difficult. For him it was actually something similar to fun, to be frank. He just took her with him into the lab, attached a small tracking device so he didn't have to keep an eye on her, but kept an ear open for any crashes or screams while he went about his business.

She was proving quite good at keeping the both of them entertained, actually. She was fascinated by all of the strange and often morbid equipment and experiments, and there was no fear of her breaking anything because of Mayuri's paranoia-inspired safety precautions.

Actually, she proved smarter than she looked- after just the first few injuries, she quickly learned what not to touch. Which was actually fairly disappointing, as that had been how she was keeping Mayuri entertained.

But alas, that didn't last for very long. After only an hour or so she got bored of looking and decided to see if she could disarm any of the security systems. When she actually succeeded in finding her way into a vat of regenerated pig organs, sacrificing his sixth-seat in the process, he was forced to call in Nemu and have Yachiru contained.

The diminutive vice-captain's intelligence was once again affirmed when, upon waking up, she not only didn't question the fact that the entire left side of her body was numb, she also remained relatively still for the remainder of the day.

Which meant that Mayuri could continue his experiments with, if not complete peace, at least a small enough amount of distraction to work.

The little pink airhead was currently sitting on a cutting board- er, surgical table, swinging her legs idly and looking through his medical journal while laughing at the illustrations.

She looked up at him, or at least the portion that was visible from behind the microscope he was looking through, and Mayuri found out the extent of her naivety far surpassed any expectations he had from someone raised in the Eleventh Division.

"Say, you're smart, right?"

He didn't even look up. "If having an IQ greater than the sum of your entire squad's can be regarded as 'being smart', then yes, I'd say I fit under that description."

"Great! Then maybe you can help! I've got this question that's been bugging me, but whenever I ask someone they get this weird look on their face and try to talk about something else."

"Interesting. I have an inclination as to what this inquiry of yours might be, but do indulge me. What is it that's been bothering you, my dwarven Mephistopheles?"

"Well, ya see, I once saw a lady with a huge beer gut, and someone said that that's what women look like when they're having a baby. But when I asked if she ate it or something, Ken-chan said a stork came down and shoved it in her for safekeeping. I know Ken-chan's always right and all, but I still wanna know where babies really come from."

"The uterus. Avian delivery-creatures don't exist."

"The what? What's a yooterus?"

"An organ in the female body, used specifically for reproduction."

"An organ? Like, your guts and stuff?"

"Precisely." He drew back for a moment to look at her, and indicated the area underneath her stomach with a gesture of his scalpel. "You have one, too. Above the urinary bladder, somewhere in this area."

"Really? So I could have a baby in me?"

"Not right now. You have to reach maturity first- at the moment you don't have any eggs to be fertilized."

"Eggs? Women lay eggs, too? I thought that was just chickens!"

"Well, it's a little bit different, but essentially, yes; women do lay eggs on a monthly basis. But unlike chickens, humans only lay their eggs once they've gone bad- if they're fertilized, they stay inside the body to develop."

"So, women lay rotten eggs every month, and if the egg doesn't go bad it stays in the body?"

"Correct. This is called menstruation, and the fertilized egg will develop into a human pupae- excuse me, infant, over a period of nine months. When the fetus is ripe, birth takes place."

"...What?"

Mayuri sighed. "Women have eggs. You understand this part, right?"

She nodded. "Uh-huh."

"When an egg is fertilized, it grows into a baby. Does this make sense, too?"

"Yeah."

"Excellent. When this happens, the baby, at this point called a fetus, grows in the uterus."

"Okay, but-"

"Let me finish first, insidious creature. When a fetus is in the uterus, the uterus expands to accommodate the growing size of the fetus. When the uterus expands, the visual result is that the woman's stomach has gotten bigger. Do you understand?"

She thought for a moment. "Okay, I think I got this- Wait, but how does the baby get in the uterus? I know about the egg part, but what's this fertilization thing that starts it?"

"Ah, of course. Sperm travels through the fallopian tubes into the ovum, or egg."

"Wha- Well, what's sperm?"

"A man's genetic material. Seeds, you could say. Excreted in the form of semen during ejaculation."

"Oh. Okay." She looks thoughtful again. "But if it comes from a man, how does it get into the woman?"

"Intercourse."

"Intercourse?"

"The act of reproduction performed by a fully developed male and female."

"Huh. Okay. So, then, when a man and a woman do intercourse,"

"The correct term would be 'have intercourse', but go on."

"Whatever- when intercourse happens, a man puts his seeds into a woman's egg, and turns into a baby after sitting in the... the... yooterus?"

"Close enough. And the correct term for 'seed' is sperm."

"Sperm? Okay. Anyway, so the baby... Uh, feet-us? Is in the yooterus for nine months, and the tummy grows to make room for it. Is all that right?"

"Very good, you catch on fairly well, don't you? Do you remember about menstruation?"

"Um, I think I got this- Once a girl becomes a woman, she starts laying eggs every month, until one of them is fermented-"

"Fertilized."

"-fertilized with a man's sperm when the woman has inter-corpse."

"Intercourse. And yes, you seem to understand as well as any child reared by Neanderthals is expected to, if not better. Very impressive."

"Yay! But... How do you know when a woman's old enough to to get full of baby?"

"The word for that is 'pregnant.' The period of incubation is called pregnancy. And this happens at puberty, at the equivalent of a human's age of twelve years or so. It's different for everyone, though. I'd say Hinamori-fukaitaichou is about that age, though."

"Really? Mousy-girl could get, uh... Preg... Pregurgitate?"

"Pregnant. And I don't know, I'm not aware of the state of her development. But that's only when you're capable of having a baby, you shouldn't actually do so for a long time afterwards. I'd say, perhaps, Nanao-fukaitaichou's age. Her biological clock is probably telling her to do so, anyway."

"Eh? What's a biological clock?"

"That only means that her body wants to reproduce while it still can- a woman only has a limited number of eggs, and when you run out you can no longer conceive-that is, become pregnant."

"Really? There's a time-limit to when you can have a baby?"

"Indeed. When you've run out of time, it's called menopause."

"Okay! Wow, no wonder no one could ever explain how babies are made, it's really complicated! I bet they didn't even know themselves!"

"I wouldn't be surprised if they didn't." He paused for a moment, then looked up from his work again at smiled at her in a manner that would have been terrifying to any person whom hadn't been in the presence of Zaraki Kenpachi for the entirety of their life. "Say, I have an idea- why don't you find Nemu and you two can go show off your new knowledge? I bet everyone would love to hear you explain it to them!"

Her face brightened up "Wow, that sounds like fun! Everyone's gonna be so impressed!" She jumped off the table, and ran off, shouting "Nemu-chaaaaaan! Let's go find people to play with!"

Mayuri smiled to himself for a moment as she left. He had done this 'parenting' thing before with Nemu, but that stupid little harlot hadn't had the educational advantage of a childhood, and he had to teach her absolutely everything all at once. And while he had designed her so that she wouldn't have a personality beyond absolute devotion, her inability to ask questions until after she'd erred had proved a point of frustration. God, how he hated it when Urahara was right.

This little girl, however, was something else entirely- she already knew the basic principles required for a human being to function, and was ready now to receive a higher education, which was really much more fun. Better yet, she was also young, receptive, caught on quickly and had the capacity to understand higher level, more abstract principles such as molecular chemistry. A child's boundless imagination allowed for understanding without question, as long as there was a minimal amount of comprehension to start with.

He decided, then, that this opened up an entirely new field of study for him. How much could he teach the hyperactive, blood-thirsty child, whom he surmised was only recently out of toddler hood? How much could he actually teach using only basic vocabulary and over-simplified concepts? If she was already a vice-captain just because of her power, what would happen if he could add intelligence, as well?

This was most definitely a subject worth pursuing. Not only would he collect data on human development and the importance of adolescence, but he would also be the de facto mentor of the second-in-command of the Eleventh division. Having an ally in such company would be most beneficial, wouldn't it? And the best part of all is that she wasn't his damn kid- he only had to put up with her during the periods of study, before she went back to torturing others.

Mayuri was practically tingling in anticipation. Finally! More live subjects! That is, uh, more opportunities to... uh... Interact with other human beings and expand his social skills. Yes.

Because referring to people as 'test subjects' was certainly inappropriate behavior not suiting a captain. Naturally.

However, while he knew that volunteering to babysit her from now on wouldn't raise any complaints, there was still the matter of her willingness to learn. He had gotten lucky today, but she probably wouldn't care about anything else he had to teach her... Unless, of course, he taught her the vital points of a human body, and all of the different techniques to kill using them. He'd have to think of what to do from there when he gauged her reaction.

Hmm, come to think of it, Zaraki probably wouldn't like all this, would he? Perfect! He had been wanting to run a few tests on the man, but hadn't been able to figure out how to get him in the lab until now. This was really all going very well, wasn't it?

And thus, Kurotsuchi Mayuri started his long journey into teaching, which, perhaps, might one day lead to him decapitating an entire classroom full of students trying to learn Advanced Physics because he decided that they were all too stupid to even be worth allowing life.

Or maybe he'd never get past Yachiru's second lesson. That was pretty likely, too.