A/N: Over on Tumblr, love-yellow-door gave me this prompt:
Molly finds something Sherlock left behind at her flat and feels a need to return it. He does not understand this need because he left it there on purpose...

It came out a bit long. Anyway, here's Wonderwall.

Sonic Love

Molly only realizes it when it's too late.

It could have happened as she grabbed for it, as she wetted it down, as she prepared it.

But no.

It's only as she walks blearily to the coffee maker that she notices. Something about the blue light cast from the machine clues her in.

Never one for bright lights at any time of the day, let alone six in the morning, she struggles to rely on the small nightlight that she keeps in her kitchen. Squinting and narrowing her eyes, she can barely make out the dark coloring on the toothbrush that is still in her mouth, rotating bristles stuttering against her gums.

Her groan of realization comes out as more a gurgle, and she struggles to suck back in the foamy toothpaste that tries to escape.

After spitting it out in the sink, she glares heatedly at the electric toothbrush that Sherlock Holmes forgot when he used her flat two nights before.

She continues to scowl as she returns to her bathroom and takes up her own toothbrush—this one a fun orange and pink, not boring black and white—and recommences with her morning toilette.

What an awful way to start the day for a person who considers early mornings to be horrid on principle alone.

She manages to cast off her tetchy mood despite her morning's ignominious start. Her day runs relatively smoothly once she's made it to work and knocked out several albatross projects in one sitting.

It's a mere five minutes before her shift's end when Sherlock arrives, eager for some—frankly anticlimactic—lab results. He even chats with her as he peruses the blood profile write-up.

"What are your thoughts for the evening?" he asks, eyes moving rapidly down rows of numbers.

Molly shrugs. "Watch a bit of telly, I suppose. Eurovision, you know."

He sneers, possibly in distaste at her plans, possibly at the completely normal thyroid stimulating hormone levels in his client's CBC. "Really? That's it?"

"It's a Tuesday night," she reminds him. "I'm hardly going out to the pubs."

"Especially since you've been trying to cut back on beer. You think it's been contributing to the four pounds you can't shed from your backside. You've been doing so unnecessarily, by the way."

Unwittingly, she cups her rear end. "Why? Because my cutting back on two beers a week to shed weight is like trying to cool the ocean with an ice cube?"

"Mmm, no," he corrects her distractedly, now googling something on his mobile. "Because your backside is quite nice and I want you to be content with your body whatever its size. Oh, it's shift change, isn't it?"

She blinks at him, stunned. Sherlock Holmes has just complimented her bottom for absolutely no reason. In fact, on his comment about shift-change, he turns and strides out of the lab without a waiting to see if she's followed him.

Apparently, he intends to stick close, however, because he's waiting for her at the staff locker room. He stands there, holding the door open and rolling his eyes at her perceived lollygagging. Deciding it's better for her peace of mind all around if she just goes with it all, Molly brushes past him and moves to her locker to fetch her bag.

They're friends. Friends compliment each other. It's the friendly thing to do.

Maybe she should tell him that he should be content with his bottom, too. That perhaps he should have an artist make a bronze cast of it….

As she stuffs her badge into one of its inner pockets, her inner monologue cuts off as she spies the toothbrush. She'd stuffed it in her canvas workbag in case he showed up at the hospital and had promptly forgotten about it.

"Sherlock," she says, turning around.

He's put away his phone and is now staring boredly at the fiberglass tiles that comprise the ceiling. "Molly," he parrots.

"You left this at mine. Just noticed it while I was getting ready for work this morning." She'd decided as she packed it that it would be kinder to omit the fact that she accidentally used it prior to her discovery.

He stares down at the proffered toothbrush, temporarily made cheerful by one of her rainbow dinosaur print sandwich bags.

"I left that by the bathroom sink," he says, frowning.

Molly nods. "Yeah, hope you have a backup," she adds with a cheerful nose wrinkle.

"I do." His expression is still rife with consternation. "Why'd you bring it here?"

"Because I figured I'd see you today?"

He waves her away. "Yes, but why didn't you feel the need to remove it from your flat?"

Unsure of whether or not this is a trick question, Molly stares at the toothbrush, too. It's situated in the bag in such a way that it looks like an acid green T-rex is chewing on the bristles.

"Because you forgot it?"

A lot of her answers are coming out as questions. Normally, Sherlock would jump on the pattern, lecturing her about tone inflection, but right now he's too busy, egads, getting upset about a toothbrush.

In fact, he looks hurt.

"I didn't forget it!"

"You didn't?"

"No!" He sighs, a man who's being tested for the exact breadth and height of his patience. "Molly, if I'm going to spend several nights a week at yours, I'll need at least a few basic things there. It's not practical to bring a duffel every time your cuddling habits and sexual appetite for me demand my presence."

He misreads her flabbergasted expression for one of extreme upset. "Not that mine for you is any less," he hastens to add. "I brought a toothbrush, antiperspirant, and a few pairs of your underwear back to my place yesterday."

"Whuh—" She's lost all ability to speak, she now realizes. She's regressed to infantile babbling.

"It's like you're forgetting our entire conversation on the matter."

At this, she regains her senses. "What conversation? When?"

He scowls. "Night-before-last. We confessed our affection for each other while I was making use of my bolthole? You came home from work and climbed straight into bed. You'd had a horrid day, you said." When she only stares, he flaps his hands. "I comforted you with a kiss on the cheek, finished perusing some old mind palace factoids, and got up to brush my teeth. When I climbed back under the covers, you said you love me and, flattered, I said I would do everything in my power to keep your love and reciprocate it." Dawning realization slackens his face. "And you were talking in your sleep, weren't you?"

Molly opens and closes her mouth, fish-style.

Instead of hustling away in mortification at her appalled silence, Sherlock comes to a decision.

"Well, now that that's settled, we can only move forward. If you want to watch Eurovision tonight, we can do so, such is my affection for you. But tomorrow I want us to play Serial Killer Guess Who."

"Just like that?" she asks weakly.

"Just like what?"

"Just like that, we're in a relationship?"

He shrugs without any of his characteristic flippancy. "Well, yes, if you concur. If you weren't saying 'I love you' to a dreamed-up rendition of that Sean Connery fellow who's so popular right now."

A giddy fluttering takes up residence in her stomach.

His lips purse with worry. "Do you concur? Do you love me?"

He doesn't have the full the question out when she nods and throws her arm around him, burrowing into him in answer.

"That's a relief," he murmurs against the crown of her head. "Otherwise it would have been hellaciously embarrassing for us to have to return our underwear to each other."

"Each other?" she asks, pressing a kiss to the underside of his jaw.

"Yes," he says, as if it were the most obvious thing in the world. "I cleared a drawer for myself the morning after our pseudo-conversation."

She'd wondered why that pile of her clothes had been set in the cat bed. They'd have to talk about respectful treatment of laundry, but for the time being, she's content to stretch up on her toes and kiss him.

After some time, they pull away from each other.

Smiling, she holds out her hand and he takes it without question, grabbing his toothbrush from her other, intent on stuffing it in his coat pocket. He pauses halfway to his destination, however.

"Molly?" he asks carefully. "Why are the bristles wet?"

She looks wildly around for a distraction. When nothing presents itself, she cries, "Why did you have get a Slim Sonic just like mine? Why not a nice, popular Oral-B? Don't you know that some people don't turn on lights until they're forced?"

It's hard to tell under the fluorescent lighting, but she would swear that the tips of his ears turn red.

He mumbles an explanation. When she asks for clarification, he says loudly, "Because they match on the countertop this way."

Her lips curl in a silly grin and he rolls his eyes. "Oh, shut up."

But he does nothing else to dissuade her goofy smile. In fact, he returns it as they leave Barts to find a taxi home.