It wasn't because the child was thin. No matter what anyone else might have thought, it was not because the child was thin.

The elderly couple whose son had gone missing ten years ago had been more than grateful for Sherlock's services, even if the search had ended with the discovery of the son's remains, and it had been interesting enough that he'd declined payment for the case. Mrs. Finnegan, however, had pressed a box of chocolate digestives on him in an effort to express her mournful thanks when they had no money. Sherlock had figured that Mrs. Hudson at least would like them.

Then there had been the child, an obviously homeless girl of seven years, and it had not been because she was thin. Nor was it because she had curly black hair, blue eyes, a dirty plaster on her knee, or because she was wearing a baggy t-shirt with the periodic symbols of Ba, Co, and N and the words "I love chemistry," on the front.

Mrs. Hudson would probably make a fuss if he gave her the chocolates, he considered, and handed the box to the child before moving along on his way home. The girl stared at him as he passed with wide eyes that protruded from her thin face, clutching the box to her thin chest with her thin arms. But it hadn't been because she was thin.

Sherlock was almost unsurprised when the girl started following him - she was homeless and seven and likely to attach to any adult figure to show even the most incidental kindness - but he didn't acknowledge her presence. Even opening the door to 221B he didn't look back, tramping up the stairs without closing it and hardly listening to it snicking shut behind the girl. He lingered in the foyer of the flat, making a significant point of toeing off his damp shoes; in the corner of his eye he saw the girl kneel down and carefully untie her fraying pink trainers.

Slowly and with deliberation he untied his scarf and removed his coat, taking enough time for the girl to finish with her laces and tuck her shoes in beside his, before walking into the kitchen. Still, he took no notice of her presence as she shuffled in after him, continuing to clutch the box of chocolates for a moment before gingerly setting it on the counter. He opened the fridge, pulled out the milk, turned, and feigned surprise. Not under his Normal guise, children were irritatingly good at seeing through that, but a more natural arrangement of his features.

"Oh," he said conversationally, "I didn't hear you come in at all."

The girl smiled, obviously pleased with her own prowess, and flushed pink around her slightly-protuberant ears.

Sherlock leaned down so he was eye-to-eye with her. "You must be my new assistant; I was very impressed by your resume, though I can't quite recall your name."

As he looked at the J-shaped pendant round her neck the girl blushed further and shifted, looking as though she'd just been caught telling a lie.

As she opened her mouth Sherlock held up a hand. "Hold on, I think I remember it, just give me a moment...Jaime?" he ventured.

The girl shook her head.

"Jennifer?"

Another shake.

"Jackie?"

Wrong again.

"Mm...Josephine?"

That one got him a giggle.

"Joanna? Jemma? Jewel? Oh, please tell me it's not Jewel."

Now with fingers stuffed in her mouth to keep from giggling any more, she shook her head again.

Sherlock pouted his lips and rolled his eyes around as though in thought. "Alright, let's see...what about Jessica?" he asked.

The girl did a little jump in place and squeaked gleefully through her fingers, nodding.

He sighed in relief. "Good, I was beginning to worry it would be something stupid, like Geronimo," he said before straightening up to put the milk on the table.

"That's a G."

"Sorry?" asked Sherlock over his shoulder, now reaching for a glass in the cupboard. He kicked out a chair and Jessica sat down.

"Geronimo," she explained in a small voice, "it has a G."

He sat across from her, crossing his arms. "So it does." Tilting his head slightly, Sherlock hesitated before holding out his hand. "Sherlock Holmes."

She shook his hand, kicking her legs idly. "Hello."

"It's a pleasure, I'm sure."

There was a tooth missing from her bottom row, toward the front. "Now, Jessica, if you're going to be my assistant - you are going to be my assistant, aren't you? Only, well, you would probably have to live here. I do experiments at all hours, being a Very Important Scientist and Detective."

Instantly Jessica nodded. "Okay. May I have a chocolate, Mister Holes?"

"Holmes, and yes you may," nodded Sherlock. Jessica hopped down from her chair and retrieved the box from the counter, bringing it back to the table before removing a digestive. Sherlock could see the pleasure on her face when she popped the sweet into her mouth.

"Now, as I was saying," he continued, "if you're to be my assistant in this Extremely Delicate Experiment, you'll have to perform an easier one first, to prove yourself. Let me show you; come with me."

They stood from the table and he walked upstairs into the bathroom, pulling a bottle of bubble bath (old experiment) from under the sink. He offered it to Jessica and she took it bemusedly. "We must find out exactly how much soap is required to produce approximately four inches of bubble foam on the surface of the bathwater. As a Very Important Scientist, you need to take water displacement into consideration, so you'll have to get in as well." He said it all with such a serious tone that the words 'in the name of Science' were probably implied.

Jessica looked at him as though he'd grown a second head. "With my clothes on?" she asked incredulously.

"Of course not! Do you think I'm insane?" he replied instantly. "I have to sterilize them; can't do a science experiment with unsterile clothes! Now, you perform the experiment, leave your things in the hall, and I'll prepare your next task." He stopped just outside the door and turned back. "Oh, and all scientists ask for help when they need it, so make sure you call for me if anything comes up."

With a wink and click of his teeth, Sherlock went back downstairs and put the milk in a saucepan and the saucepan on the stove. When he heard water running, he treaded quietly back up the stairs - skipping the fourth, it squeaked - and sat just out of sight of the cracked-open door, listening for signs of trouble while Jessica performed her experiment. She hummed to herself as she poked her dirty disheveled clothes out into the hall. Sherlock picked them up and, as an afterthought, knocked on the door. "Jessica?"

The humming stopped. "Uh-huh?"

"You need to test the density of the bubbles as well as the depth," he said through the door.

"What?"

He could hear the sloshing of water against the sides of the tub and took that to mean she was safely in the bath. "Can I come in, are you covered up?"

There was a pause, and then the strange sticky noise of bubbles being pushed around. "Uh-huh," she agreed. When Sherlock cautiously opened the door it was to see her sitting innocently in the middle of a veritable mountain of soap bubbles, completely covered from the neck down. The corner of his mouth felt curiously tight and stretched; there were bubbles on her left ear.

"The density," he elaborated, kneeling beside the tub. "You need to test it, like so:"

He scooped up a handful of bubbles at the edge of the tub, wiggled his fingers to let a few drip back into the tub, and then blew on the suds until they flew into the air around Jessica's head. She giggled delightedly and copied the action, blowing bubbles into Sherlock's hair. His mouth tightened mysteriously in the corners again. He set her clothes down behind him on the floor and picked up the bottle of shampoo. "Now, part of being a Very Important Scientist is having clean hair, so we'll take care of that since you're already in the tub doing this experiment, shall we?"

Jessica nodded so frantically that the water began sloshing about again.

"Good, now close your eyes; we'd hate to get this in them, eh? That's another experiment altogether..."

Sherlock had never done anything like this before, caring for a child, but found the tightness in his mouth was directly correlated with doing something that made Jessica laugh, and that it was accompanied by a similar tightness in his chest. Recalling something his mother used to do when he was small, he wetted Jessica's hair, lathered in the shampoo, and then twisted it in his hands until it looked like she had horns and found a hand-mirror to show her. "This is extremely important," he said gravely, "you'd better remember this for later."

She shouted with laughter, further confirming the correlation.

They performed her first experiment with gusto, blowing bubbles around the tiny bathroom until every surface was oily with soap and he'd shown her how making the thumb and forefinger into a ring could make individual bubbles as well. Then she insisted on trying the horn experiment on him, and he'd had to stick his head in the water, likely ruining his shirt. After that little fiasco he'd stuck a sponge in her hand and made her scrub everywhere he knew it wouldn't be proper to if she ever were to mention the funny man who'd given her candy and a bath.

Sherlock pulled the drain and instructed her in how to rinse off with the removable shower head, told her where the towels were in the closet, and then took her clothes downstairs to the wash machine. He then went to his and John's bedroom and fetched a clean t-shirt and pair of shorts that John had found hilarious and bought (in the full knowledge that Sherlock would never wear them). "I'm leaving your official Very Important Scientist uniform out here in the hall, Jessica," he called through the door.

For the record, it hadn't been because she was filthy, either.