Sherlock had an oral fixation. That was the only possible explanation for why he found so many bloody excuses to put things in his mouth. John grimaced as he watched his flatmate extend his tongue to delicately lick the dead woman's purse.

"Doesn't that contaminate the crime scene?"

Sherlock snorted dismissively. "It's hardly going to ruin anything the Yard would have actually tested. Alkaline taste, though - not bleach, would have smelled that, but she put this purse down somewhere that had been recently cleaned with industrial-strength chemicals without added odors. Seems obvious she was inside the building at the time of the murder."

Lestrade frowned. "Her purse tastes bitter, so she's our killer?"

"Not remotely, and do try to keep up," Sherlock snapped. "She was inside. As was someone who was cleaning. Find your housekeeper and you'll find your murderer. Based on the dead man's office, I'd say he was dodgy about getting out paychecks on time, and seems the type to have been handsy as well. This woman was just collateral damage - witness, perhaps." He turned on his heel and swooped away from the body, coat billowing out behind him. "That's all you need from me - come on, John."

John and Lestrade shared a commiserating look before John gave up and trotted obediently after him.

The licking thing was really getting out of hand. John could ignore Sherlock nibbling on pens, pencils, toothpicks, and the like, but it was bloody hard to concentrate on his book when Sherlock had microscope slides strewn all over the kitchen and was licking each one in turn.

"Should I even ask what you're doing?"

Sherlock looked up, startled out of whatever-it-was he was experimenting on. "Thinking," he said.

"Looked like you're slobbering on your microscope slides, to me."

"Experiment."

"Ah. Of course." John shot him a tight smile and tried to focus on his book again.

"Residual effects of various brands of chewing gum and breath freshener on saliva samples over time . . ." Sherlock trailed off, staring at John. "Oh. Oh."

"What?" John glanced down to see if there was something wrong with his clothing. But no, nothing out of place-

"I'm bothering you," Sherlock said.

"Hey, when you're in a strop, you complain that my breathing offends you," John countered. "I hardly think you're one to talk."

"No, the - the licking thing. It bothers you."

"Bother" isn't the word I'd use . . . John shrugged instead. "A bit unsanitary, perhaps, but I'm not touching your microscope anyway. None of my business."

"You watch me. When I'm thinking and I chew on my fingertips."

John shrugged again. He couldn't really deny that - Sherlock did have a habit of tenting his hands under his chin as he thought, usually when he was flopped any which way on the sofa, and he often ended up resting his front teeth on his fingers. Sometimes he had both pointer fingers inside his mouth, all the way to the second knuckle, and when he drew them back out again they'd be red with tooth marks and shiny with saliva. He had such a glorious mouth, mobile and expressive, and John tried very hard not to think about what else Sherlock might like to jam in there.

And that was the problem, wasn't it? John tried not to notice, tried not to think about Sherlock that way, but Sherlock stuck things in his mouth all the bloody time and never seemed to have the slightest bit of shame about it. If anything, he reveled in the tastes and sensations - for someone who ate as little as Sherlock did, it was astounding how often he managed to find excuses to lick or suck on or nibble on things. John could hardly be blamed if he started daydreaming about that expressive mouth.

Not that he was gay, by any means. Gay would indicate he was interested in other men too, right? Being straight was comfortable, it was normal, it was expected. And if John happened to have a thing for Sherlock's mouth, that didn't necessarily mean he was gay. Sherlock just had . . . a particularly gorgeous mouth. Fuckable, John's brain helpfully supplied, and he squirmed in his seat.

Sherlock, of course, noticed, and abandoned his experiment to come sit in his armchair a few feet away. "Fascinating," he murmured.

John tried very hard to ignore him, keeping his eyes strictly downward on his book.

"I am aware I use my mouth more than most people," Sherlock said quietly. "I think better when I suck on things."

John could feel his face flame, but if he could just pretend long enough, Sherlock might drop it-

"I'm told I'm rather good at it, actually," Sherlock continued. He ducked his head so he could make eye contact even though John steadfastly refused to look up. "I've had the most amazing epiphanies, given the right things to suck on."

"Right then." John slammed his book closed. "I'm going upstairs."

Sherlock said nothing, just stared after him with calculating eyes and his lower lip caught artfully between his teeth in a way that made John suddenly very glad he was wearing somewhat baggy trousers. He escaped up to his room, flopped face-down on his bed, and groaned.