Disclaimer: I do not own any part of the BBC Sherlock world and make no money from this story.

It starts with a petulant foot being pressed in to the side of John's stomach. John has just made a joke at Sherlock's expense as they sit on the couch watching a movie. Well, John is sitting on the left side of the couch and Sherlock has stretched himself across the remaining part, feet towards the other man.

When the foot makes contact, John jumps and giggles for a few seconds before his face falls suddenly serious and he glares at Sherlock, "Stop it."

Sherlock looks at him like a piece of a puzzle has just fallen in to place, "Are you ticklish?" he asks with a twinkle in his eye.

"No," he lies. Poorly.

Sherlock's foot finds the same spot that caused the reaction previously and pokes it gently. John giggles again before turning serious and grabbing his foot to hold it away from his body, "I swear I will rip it off," he snarls.

Sherlock is far from feeling the intimidation that John would like him to. He grins widely as he pushes himself in to a sitting position close to the other man, his foot being released in the process.

"Where else are you ticklish?"

John looks at his face and sighs, "Stop looking at me like I'm your next experiment."

His eyes light up, "Would you let me?"

"No!" John says before turning back to the movie and trying to ignore him. It's futile now, though; Sherlock is hooked on the idea.

"John, please," he begs with excitement, "I have no database on reactions to tickling in my Mind Palace; no one has ever allowed me to test them."

John raises an eyebrow as his attention is drawn back to him in surprise, "Why the hell do you even need a tickling database?"

He shrugs, "Same reason I need a sneezing database."

"That really doesn't answer my question at all."

"Science, John!" he exclaims a bit exasperatedly, "Physiological reactions to outside stimuli."

His mouth drops open, "You honestly believe the utter shite that comes out of your mouth, don't you?"

Sherlock's head tilts to the right in confusion, "Of course I do."

John sighs heavily, "You're not going to tickle me for science."

"Please," he begs again, "don't you trust me?" And he does look honestly hurt.

"Of course I trust you. I've just spent my entire life having people constantly taking advantage of the fact that I'm so ticklish because they think it's funny. It's not a fun thing for me."

"I don't think it's funny; I think you're fascinating," he asserts and John blushes slightly, "I've never been remotely ticklish in my entire life."

"You're lucky then," John grumbles.

"I promise that I would try to make it as quick of an experiment as I can."

John is caught by the honestly curious and hopeful look in his best friend's eyes. He knows that Sherlock has never really been close to anyone before, so it doesn't surprise him in the slightest that he's never gotten the opportunity to learn something like this. He knows that his answer was already decided before he was even given the choice: when it comes to this man he has slowly fallen in love with, he can have all of him.

John sighs in defeat, "Alright, but I really can't be held responsible if I end up hitting you," he smirks, "it's become an unfortunate natural reflex."

"Really?" his eyes brighten again, "that is fascinating!"

John chuckles, "You would think that. So, how do you want to do this?"

"What would make you most comfortable?"

John's eyebrows rise again at the consideration he's allowing for, "Honestly? I'd rather stay sitting; it will help me feel more in control."

John can practically see the database forming even now, the first piece of information being logged.

"That sounds fair. Where are you ticklish besides your right front side?"

"Practically everywhere," John says in resignation.

"Let's start at the top," Sherlock says matter-of-factly and they do.

John walks him through spots that Sherlock then touches to illicit a reaction that ranges anywhere from a slight exhalation with a grin to a full guffaw and difficulty breathing. There's just below both ears, the juncture of neck and both shoulders, armpits down to mid-rib, right front side, left back side, front and back of both knees, and of course his feet. Through the experiment, Sherlock considers himself lucky to only be punched in the arm once (when he touched the back left side) and kicked only when touching his feet.

Through the entire experience John naturally drew away from the younger man's probing hands to the corner of the couch, causing Sherlock to follow him and practically hover.

"That's all of them," John pants from the exertion.

He looks at Sherlock's eyes – really looks at them - for the first time since the experiment started and is stilled by the desire and wonderment he sees there.

"Sherlock?" he asks quietly, almost afraid to break the spell that leaves him desperately wanting to tell the unspoken barrier they've been hiding behind for years to kindly go fuck itself.

He sits upright, allowing John room to breathe (not that he wants it) but not looking away from his eyes, "You're so open and giving," his wonder shines through his voice now, "so trusting," he adds as John moves to sit straight himself, "you allow yourself to feel everything."

And before John can even register that he hasn't stopped moving forward, his lips have found Sherlock's. As they groan and Sherlock pulls him closer, John naturally shift so that he is straddling his lap; not quite close enough yet for them to feel how aroused the other is.

As Sherlock unconsciously runs his hands down his previously ticklish sides, John moans as his body seemingly propels itself fully in to Sherlock's of its own accord.

They break from the kiss simultaneously, looking at each other with shock at the revelation. John doesn't need to hear the question that Sherlock's eyes are clearly asking, and he hesitantly nods while biting his lip.

Sherlock smiles affectionately at John before moving his lips and hands over the previously tested territory, logging the differing reactions now that the situation has been completely altered.

When he kisses below his ears John sighs and moans quietly.

When he kisses the juncture of neck and shoulder John moans loudly and his hips move in circles to become closer to Sherlock's heat. They both groan as the action brings their arousals in to contact through their trousers.

When he runs his hands down from armpits to mid-rib once more, the reaction is exactly the same as before: his entire body presses forward to be closer to him.

When he runs his hand over the front right side John's whole body rolls as he pulls away.

When he runs his hand over the back left side John moans loudly again as his hips thrust forward in to his answering hardness, causing them both to moan again.

When he runs his hands over the front of his knees they pull closer to Sherlock's body and places more space between their hips.

When he touches his feet, however, he barely catches John before he falls backwards off of his lap with the same guffaw as before.

"I'm sorry," Sherlock chuckles as John glares at him, "that one's the same. Won't happen again," he promises before moving in to kiss the pout from John's lips.

As Sherlock realizes that their hips are no longer connected he quickly develops a plan to use his newfound knowledge of John's responses to play him like a violin.

He runs his hands down John's sides, causing his body to close most of the distance between them in one move. Then he repeatedly touches the back left side that causes John to thrust his hips forward and rubbing their erections together once more. Sherlock growls as he suddenly moves his mouth to the right side of John's neck to suck and gently bite the place that causes his hips to move restlessly faster.

"Oh my God, Sherlock," John gasps as he brings his right hand to tangle in his curls, unaccustomed to feeling so much pleasure wash through his body at one time.

Sherlock hums in encouragement and bring his left hand to John's hip to help pull him closer. It takes mere minutes before they're both on the verge of coming in their trousers without so much as a hand to aid the friction.

"Sherlock, please," John moans desperately, " please."

In response, Sherlock finally lifts his hips to meet John's a few times and it brings them both over the edge as they moan their completion together.

As they hold each other, panting with their foreheads together, John begins to chuckle.

"What?" Sherlock pulls back warily, an unsure look in his eyes.

"I've never experienced anything like that before. You're amazing," he says before pressing their lips together briefly.

"I'm amazing? You're the one whose ticklish spots turn in to extremely potent erogenous zones."

"I can say with absolute certainty that that fact has never been discovered before your genius brain figured it out."

"You know, all scientific studies should be performed multiple times to be proven accurate," he smirks in response.

"Right, I always forget that rule," John feigns seriousness, "I know it's not your typical procedure, but maybe we should clean this experiment up before starting a second," he says with a pointed look to their trousers.

Sherlock rolls his eyes, "I always clean up my experiments."

John scoffs, "You absolutely do not!"

"Well okay, technically you're right, but I think cleaning this one will be fun. Shower?"

"Oh God yes," he beams as he stands and drags Sherlock to the bathroom.

A/N: Thanks for reading! I'd love to hear your thoughts and/or constructive criticism!