In retrospect, it was a very bad idea to introduce Sherlock to online pornography. He monopolized John's laptop for the rest of the day and - based on the sounds coming from the living room long after John attempted to go to sleep - overnight as well. John was greeted the next morning by a cheerfully sleep-deprived Sherlock and a dead laptop battery.

"I've been researching all night."

John gave him a side-eye and shuffled into the kitchen to put the kettle on. With any luck, by the time he finished in the loo, the water would be boiling and he'd be that much closer to tea. And he'd have something to do while trying to avoid thinking about Sherlock watching porn as "research."

"I found your browser history interesting, but not as helpful as I'd hoped."

Christ, he'd - but of course he had. Because Sherlock had no boundaries. "I suppose it's too late for me to tell you that's a bit not good, Sherlock. I've given up on trying to keep you from using my laptop, but porn viewing is a rather private activity and I'd like to keep it that way."

"Why is it private?" Sherlock looked honestly confused. "You shared one of your preferred sites with me last night."

"I - hell. Let me go use the loo first and brush my teeth. I'm not awake enough to be having this conversation with you."

John felt much better without morning breath. By the time he re-emerged, the water was boiling and Sherlock was getting out the tea and two cups in what was a blatantly manipulative attempt to encourage John to not be mad.

"Tea, Sherlock? Really?"

Sherlock flashed him an inscrutable look over his shoulder. "I figured you'd be more willing to give your opinion on a few of the videos I found if you had something concrete to do with your hands. Holding your tea works."

"Christ," John grumbled under his breath, not caring that Sherlock probably heard him anyway. And then, louder, "why do you think my opinion would matter?"

"Because you're a doctor, you watch pornography yourself, and you also have a penis so at least the physiology ought to be the same." Sherlock dumped an ungodly amount of sugar into one cup and sloshed a much more reasonable amount of milk into the other. "As you said, you're the expert."

"That was sarcasm." But John picked up his tea and followed Sherlock back to the living room, where he had set up (his own, for once, probably because John's was currently charging) laptop at the desk and arranged two chairs for optimal viewing. John perched on the edge of his chair, ready to jump up and go hide in his room if necessary, and waited while Sherlock pulled up the browser and opened a bookmarked link.

"This one got the largest physiological reaction," Sherlock said over the unmistakable moaning and gasping coming from the laptop's speakers. On the screen, a tightly-muscled tattooed man was tied to a padded bench and was being methodically flogged by a smaller but no less fit man with rainbow-colored hair. "I tried to catalogue all the variables - search out videos which included only the sex, only the restraints, only the flogging, only the tattoos, and so forth - but observational studies don't lend themselves well to creating a control group."

John coughed, hiding his expression behind his hand. Only Sherlock. "Yeah, this really isn't my area, sorry."

"Some of the videos in your search history had similar elements," Sherlock pointed out.

". . . Not the same thing."

"I can't analyze these properly, John. The extreme multicollinearity prevents all but the most cursory of conclusions."

John leaned back and crossed his arms. Sherlock probably wanted him to ask what multi-whatever meant, but he didn't feel like rising to the bait. "And what conclusions are those?" he asked instead.

Sherlock didn't answer right away, instead fiddling with the laptop and bringing up another video - this time of a man being spanked by a tall woman dressed completely in skin-tight leather. "This one elicited a rather elevated response, too," he said. The double-entendre went right over his head, but John had to fight to suppress a completely inappropriate schoolboy giggle.

"So . . . you're into BDSM. Okay."

Sherlock shook his head. "Not the sex - I was able to eliminate that variable on its own; there's a good variety of examples to choose from. It's something else." His cheeks reddened a bit. "Intellectually I know I want to delete whatever is causing my reaction, but . . ."

"But you enjoy it." John adopted his most patient humor-Sherlock-because-he's-missed-something-really-basic voice. "Look, sex feels good. Getting turned on, fantasizing, actual contact with someone else - it's all neurochemical. There's no reason you should feel like you have to hide all that - it's part of being human."

Sherlock shot him a quelling look. "I don't use my body like that."

"Well it's about time you started." John sighed. "Look, odd as it is to suggest, have you tried calling Irene Adler? She's more of an expert on this than I am, and you said she was the one who started this whole line of questioning in the first place. Plus, BDSM is kind of her specialty. You said she's not a threat to the crown, she clearly fancies you, and I bet she'd be on board if you wanted to approach this empirically."

The sudden brightness on Sherlock's face was nearly blinding. "Brilliant, John!" He slammed the lid of the laptop closed, instantly silencing the video. "Here, use my phone. Her number is in the most recently received texts."

John caught the phone on reflex. "Hey now, I wasn't volunteering -"

But Sherlock wasn't listening. "This will be perfect," he muttered to himself, already lost in Sherlock-land. "A chance to craft a controlled experiment, a research aide experienced in the subject matter, and a way to tweak Mycroft's nose at the same time. He'll be furious."

Yeah, because Mycroft is exactly who I want to be thinking about in this context. But the only way to make Sherlock to shut up about anything his laser-focus landed on - including porn, apparently - was to allow him to get it out of his system. John dialed.

"Hey handsome," Irene's voice drawled from the other end. "So you do know how to use a phone."

John cleared his throat. "It's John Watson, actually."

Irene hummed. "My greeting still applies."

Fuck, what do I . . . ? But Sherlock was staring at him with large, hopeful eyes, and John recovered quickly. "He's making me call - it feels like third form all over again - but Sherlock's discovered a sudden interest in BDSM and wants your help in figuring out what it is."

The line was quiet for several seconds. "Hand him the phone," she said finally. "This isn't the kind of thing you negotiate by passing notes in the hallway."

John dutifully passed the mobile to Sherlock.

"You need my agreement in person?" he grumbled into the phone. A pause. "Yes, exactly. An experiment - I haven't been able to tease out the proper variables." Another pause. "Nearly none - you're not far off." Longer pause. "Yes, that sounds perfect. Yes, he'll come too. No, I know exactly where it is. Tomorrow evening, then."

John stared at his flatmate as Sherlock hung up the phone. "Please tell me you didn't just promise Irene Adler that I'd come with you to a creepy sex dungeon."

"Of course not." Sherlock smiled blandly. "She works out of a two-story house just outside the M25 - rather nice area, actually. Not creepy at all. I suspect because there's enough space between buildings to muffle any suspicious noises. You're free tomorrow at six, right?"