Josephine had discovered the skull.

John didn't know what had drawn her attention to it, because she'd never seemed to notice its presence before – or its absence, when Mrs. Hudson took it away, as she routinely did. He'd never been certain if their landlady took it because she objected to it or because she enjoyed talking to it in the same way Sherlock did. He'd never asked – it was Sherlock's property and he always wrangled it back.

But Josephine had clambered onto the couch and pointed to it.

"Jah!" she said happily, her word for John since she couldn't manage the actual name. It was closer than what she'd initially called Sherlock, because she had started with just a hissed lengthy "s" sound for him, followed by a collapse into giggles. Lately, she'd manage "Ock", and John knew that, whether Sherlock liked it or not, he was going to end up as "Uncle Lock" for the rest of his life. John had taken to calling him "Lock" under his breath to annoy him, but had dropped it off when Sherlock had responded one day with a very pointed "Yes, Johnny?". Nicknames were then confined to Josephine's budding English, and Tricia, with whom John could not contend. Sherlock still resorted to "my dear Watson" at crime scenes, and John despaired of ever having a nickname for his husband, a man on whom even something like "love" or "honey" would stick like snow in a desert. He toyed with more embarrassing nicknames, but only to himself, because he wasn't ready to put up with "Johnny" from Sherlock, who would, John knew, adopt it in a heartbeat if given half a reason.

He was impressed he'd made it so far without it, actually.

"It's a skull," he told Josephine.

She nodded brightly, pointing at it, then at herself. Although Tricia and Henry had never taught her formal baby sign, Josephine had adapted several gestures rather well, and John was beginning to suspect that she was going to be very bright, perhaps problematically so.

He'd mentioned her intelligence to Sherlock once, who had stared at John like John was mad, and said in a tone that told John precisely what Sherlock thought of his observational abilities:

"Obviously."

As though he'd always known.

They took her about once a week, and the occasional Saturday, now that Tricia was back to work full time, and Henry's schedule had never really taken a break. They made their schedules work well enough that one of them was generally at home with Josephine during the day, although she went to a sitter Mondays and to John and Sherlock's usually Thursday evenings. John didn't mind; he was happy to have a niece, and someone else around the flat. Mrs. Hudson adored Josephine, of course, and found excuses to come by when the baby – toddler really now, since she was just a few weeks shy of her first birthday – was there.

Even Sherlock liked having her there.

That was wrong, John thought.

Sherlock loved having her there.

It was a constant mystery to John. He'd never seen anyone so obviously unsuited for interacting with children as Sherlock, or so he'd thought before Josephine had been born. The man was almost completely unaware of the needs of others – except John, to be fair – kept his own schedule, or lack thereof, had almost no set meal habits to speak of, couldn't contend with what was a very broad definition of idiocy in other people, and disdained almost everything he deemed boring or unimportant, which was almost everything.

But he'd been smitten with Josephine since the first moment he'd held her, and their niece returned the affection with enthusiasm. She followed him about the flat from the moment she'd been old enough to crawl, and had been fascinated with him long before that.

It wasn't that Sherlock tolerated this. He actually enjoyed it, and would stop working to play with her, take her out, talk to her incessantly, read to her from his own books – which were far too old for her, although Sherlock objected to that, saying that she clearly wasn't being given enough credit.

He'd even turned down a case once, when they'd been watching her one Saturday.

Lestrade had rung and Sherlock had answered, listening to the details of the case, nodding along, then had said:

"So sorry, too busy."

And hung up.

Lestrade had come by less than half an hour later, red faced and furious, not at all used to being turned down by his sole consulting detective. When John had let him into the flat, Sherlock had looked surprised.

"I told you I'm too busy," he'd pointed out.

"Doing what, precisely?" Lestrade had demanded.

Sherlock had just pointed to Josephine, as if it were an explanation.

"You're babysitting?" Lestrade had asked, disbelief replacing the frustration on his features, turning to John for some indication that this was not the case, that Sherlock was playing him.

"Excellent powers of observation as always, Lestrade," Sherlock had replied lightly. "I can see that you made detective inspector for good reason. Send a car round tomorrow and I will see what I can do."

Seeing what he could do involved solving the case and getting a suspect apprehended in four hours, so Lestrade had lost his reasons to be upset. John didn't think the DI was ever going to get over Sherlock's change in priorities.

He still caught Lestrade watching Sherlock on cases, as if trying to figure out if the detective was going mad.

Or, possibly, going sane.

John gave up trying to understand it. He'd never seen a baby react to any adult the way Josephine reacted to Sherlock. Other than the constant following him about, she actually listened to him. In a way she didn't listen to either of her parents or John or any other authority figure.

She'd got into one of their kitchen cupboards one day – thankfully not one where Sherlock stored his equipment and supplies – and had pulled all of the tins and packages out before John realized what she was doing, since he'd left Sherlock to keep an eye on her so he could shower. Sherlock had been absorbed in fiddling with some experiments so he could get the results he'd anticipated and publish some paper. John had come back in, stared at him, stared at Josephine, and then tried to get everything put away, daunted by her persistence in taking each tin out again as he put it back. He was just about to bodily move her, knowing it would result in howling, when Sherlock leaned back in his chair, met her eyes, and said firmly:

"Jo. Put it away."

She'd obediently and very happily put everything back, shoving it in haphazardly so that John had to rearrange it later, but doing a complete job of it.

John was currently operating under three theories about Sherlock and Josephine.

One was that Hogwarts was real and Sherlock had gone there and was a full-fledged (probably dark) wizard who could control the minds of small children.

The second was that, through a series of either alien and/or secret government experimentation, Sherlock was actually Josephine's biological father and this was some underhanded plot to populate the world with Holmes-level intelligence. Mycroft fit nicely into that theory, too. He might even be behind it.

The third was that they were alike enough as to be linked telepathically. This wasn't a very satisfactory explanation, either.

He was fairly certain Josephine would not turn out to be a sociopath, given what living with one had taught him. Once, John had stubbed his toe moving something while watching her, and, while he was trying very, very hard not to curse in front of her, she'd toddled up to him, put a tiny hand on his cheek, and kissed him clumsily on the eyes. It had made him laugh, which helped the pain. She also liked to cuddle with him and Sherlock, especially when she was about to fall asleep or when she was being read to, so she didn't shun the physical contact that Sherlock did with most people except John, and to a lesser extent, Tricia – and, of course, Josephine herself.

She pointed to the skull again, looking mildly irritated that John wasn't doing what she wanted.

"It's not a toy, sweetie," he said, shaking his head.

"No!" she protested, gesturing to it again. John wavered; Sherlock was home, but he was in the shower. A few minutes wouldn't hurt, would it?

Carefully, he took it down and put it on the couch between them. Josephine plunked herself down immediately, absorbed, reaching out to touch it, then giggle and pull her hand back. She leaned over, peering into the eye sockets, then traced a finger over one of the cranial sutures, grinning up at John.

"Careful," he warned her lightly. She nodded, as if accepting this caution, and tapped on the skull's teeth, then her own.

"Well done," John said. She stood up carefully, tapping John's lips, so he bared his teeth for her and she tapped those quickly instead.

"Teeth," he said.

"Eet," she replied.

"Close enough," John agreed.

Footsteps coming into the living room startled him and he looked up guiltily, aware that he'd been caught. Sherlock was dressed, but his hair was still damp, more curly now than it was dry. John always loved that, and wondered if he could distract Sherlock with that fact, to keep him from seeing he was using the skull as a toy.

"Ock!" Josephine said, pointing down at the skull. Well, no hiding it now.

"I see," Sherlock said and John opened his mouth to protest his innocence, that he was defenceless in the face of his niece, but Sherlock wasn't looking at him, nor did he seem concerned. John closed his mouth again, brow furrowing. He usually let Mrs. Hudson get away with stealing it, but she wasn't liable to break it.

"Where's your frontal?" Sherlock asked. John blinked, but Josephine grinned, jabbing a finger into her forehead, then tapping the same place on the skull. "And your mandible?"

She tapped her chin, then sat down to do the same to the skull.

"Very good," Sherlock said and she grinned, clapping her hands for herself.

"Sorry," John said. "You're teaching her anatomical terms?"

"Why not?" Sherlock replied. "There's no reason she can't learn them. She's quite an adept student, as a matter of fact. Far superior to many university students, certainly."

"And you're teaching her with the skull?"

"Yes, of course," Sherlock replied, looking mildly surprised at the implication that he wouldn't use the skull for instruction. "It's a far better tool than a living human head, which doesn't show the sutures or the differentiation between the bones. How else is she going to learn where her zygomatics and maxillae are? You can't feel those sutures on a living person, but you can see them quite clearly on a skull, John."

John only raised his eyebrows, shaking his head. Josephine was back to examining the skull again, pushing it gently over onto its side, then trying to squint into the ear canal.

Definitely theory number two, John thought to himself. Sherlock displaced John on the couch, picking up the skull, and resumed the instruction he'd apparently been giving his niece. John shook his head, grinning to himself, and left them to it, disappearing into the kitchen to make himself a much-needed cup of tea.