Author's Note: This is something I've had lurking in my brain for awhile now, but it took some time for me to get into the right frame of mind to be able to write it. It's been a long while since I let my more absurd side come out in my writing.

I know very little about how the loci technique actually works, so I apologise if any of this seems implausible.

Enjoy the brief glimpse into Sherlock's mind palace!

Purple-Tinted Glass

It was a fairly typical sort of afternoon at 221B Baker Street. Sherlock was to be found stretched out on the sofa in one of his numerous dressing gowns, his eyes wide and strangely blank as they stared up at the ceiling. His lips were moving rapidly as though speaking words which only some invisible presence could hear – the only sound that issued from them was a soft, blurred hissing, like the whisper of a sudden jet of steam. His hands were jumping through the air above him as if trying to conduct an orchestra that could only read about two thirds of the music they were attempting to play. The occasional frown that crossed his features, followed by a negative jerk of his head, seemed to reinforce that image.

Sherlock didn't even acknowledge the presence of his flatmate as the other man ambled in, hair damp and mussed from a long sojourn in the shower. John casually twitched that day's newspaper from the desk and dragged it along with him into his usual armchair, settling down with an easy groan of relief. Sherlock spared a fraction of a second to feel grateful that his friend seemed more inclined to re-establish a sense of physical comfort than attempt a conversation; then he pulled all his focus back inside again.

He had a case to solve.

Cat. Felis (silvestris) catus, of the family Felidae. Lion tiger leopard serval ocelot lynx cougar cheetah…

It had to be about the cats, it had to. There was nothing else to go on and there had certainly been enough of the creatures wandering the little cottage.

Cat(herine) Kat(herine) Çat (Erzurum Province, Turkey) Cat (Kingdom of Cait)cat (Zodiac, Vietnam)…

And it wasn't just the prolific number of cats, either. That wouldn't have been so odd in itself, there was always that stereotype of the "crazed cat lady" who never married and instead lived in isolation with at least half a dozen felines and always managed to find room in heart and home, somehow, for just one more. No, the strange thing in this case had been the obvious mental obsession with cats. Cat pillows, cat statues, cat crockery, cat bed sets…

Cat(egory) .cat domain concatenate program (Unix) cat state…

"Sherlock?"

What was so important about cats anyway? Why go to all the trouble of proclaiming such an obsession? Had someone taken offense to the cat-centric world that began on the threshold of the cottage?

"Sherlock."

Catalase chloramphenicol acetyltransferase CAT CAT(k) CAT scan…

"Sherlock."

Cats (Andrew Lloyd Webber) CATS (software) CATS pipeline….

"Sherlock!"

Something collided with the side of his head, something lightweight, thin, cylindrical, and rather pointy on one end. It took a moment for Sherlock to register that John had chucked a pen at him. He immediately dropped his still-conducting hands and snapped his head around to look at John. The other man looked right back with an expression that was part annoyance, part amusement.

"What?" demanded Sherlock loudly, startled from his train of thought and very irritated by that fact.

John shrugged. "Just wanted to make sure you were still in there," he said blithely. "You haven't moved since we got back. Well, aside from this –" He did an absurd imitation of Sherlock's hand movements.

Sherlock glared levelly over at him, his jaw tight. "You interrupted me to confirm something you already knew?" he said dangerously.

John didn't seem to be aware of that danger – or else he didn't care."Oh, sorry," he said lightly – blatant lie. "Were you in your mind palace?" The slight way in which he emphasised the phrase made it clear that he still found the concept a bit of a laugh, no matter how effective Sherlock had proved that technique to be. He leaned back in his chair, idly turning a page of his paper.

Sherlock continued to eye him with an increasingly unpleasant look. The little smile on his flatmate's face – no, smirk was more accurate – was that strange compression of the lips that only John could manage – a bit sarcastic and speaking volumes. Sherlock recalled a phrase from a book he had once read; one character had given the sort of smile that made another character want to hit it with a brick.

At the moment, John had a very brickable smile.

"Yes," he replied, with the sort of look that could have easily withered half an acre of very resilient corn stalks. He turned his face to the ceiling again, making his way back to the point at which John had felt the inexplicable need to interrupt.

"Ahh."

Silence – wonderful silence – fell for a minute.

"What's it like?"

Sherlock inhaled sharply, twisting his head around once again. The look he gave John this time would have gotten several acres of corn and probably the nearby runner beans in the process.

"What is what like?" he breathed out, his tone just daring John to answer.

"This – mind palace of yours." John made a gesture that was evidently supposed to indicate a large structure, though in what subset of sign language, Sherlock couldn't imagine. "You've never really talked about it, seems like it's just sort of there when you need it."

"Mind palace. Mind. That usually implies thinking, which is generally more difficult to do when one is talking, as you are currently proving to me – though you seem to be having a fair to poor go at it yourself right now."

John smiled the brickable smile again. "Subtle but unamusing, Sherlock," he murmured, glancing at the newspaper again before tossing it aside. He leaned forward slightly in his chair. "No, seriously – what's it like? I mean, what does it look like, for starters?"

Sherlock groaned inwardly as he saw the expression of intense curiosity on his friend's face. Why did it matter what his mind palace looked like? It wasn't as though John was going to be able to see it. On the other hand, John was clearly intrigued, somewhat of an unusual occurrence, and it meant that he probably wasn't going to let this go until Sherlock answered.

The consulting detective kneaded his forehead resignedly with one hand. So much for thinking. He didn't have any intention of cooperating fully, though, no matter how much interest John showed.

"What do you normally think of when you hear the word palace?" he asked finally, thinking it as good a place as any to begin. Better to get rid of any delusions before building up a more accurate picture.

"Erm…" John thought for a moment. "Something Oriental, probably. Like, Taj Mahal?"

Typical. Just typical. "John, the Taj Mahal is a mausoleum, not a palace."

"Yeah, well," replied John, sounding defensive, "I meant the way it looks – the towers –"

"Minarets."

"– and the domes –"

"Chattris."

"You know what I mean!"

"Do I?"

"Yes!" Now it was John's turn to look annoyed.

"An architect would be confused, you know," Sherlock pointed out.

"Yes, but you're not a bloody architect, Sherlock."

Sherlock smirked. "I built my mind palace, didn't I?"

"Oh, God." John dropped his face into his hands. "Sherlock, just stop making wisecracks and showing off how clever you think you are for five minutes, okay?"

Sherlock looked slightly ruffled at that. "I don't think I'm clever, John, I know I am."

"Alright, we know, Sherlock." John looked up again with a note of exasperation in his voice. "Just – put your ego on standby for a few."

Sherlock had a retort ready, but then thought better of it. Continuing to argue a moot point wasn't going to make this little interrogation session go by any faster, he realised. Consequently, he subsided into a dignified silence, waiting for John to speak again.

"So," said John eventually, when he had apparently deemed it safe to resume the main thread of conversation again, "your mind palace – is it more like a castle, then?"

Sherlock sighed. "There are many types of castles, John."

"Okay, well… has it got towers? Seems like you would, I dunno."

Sherlock exhaled loudly before replying. "Yes, it has towers, of sorts."

"Right." John nodded, his face screwed slightly as though trying to visualise. "How about a moat?" he suggested cheerfully.

Sherlock covered his eyes with his hands for a moment. "No," he said, very distinctly, "there is no moat." He was already regretting getting into this conversation.

"Shame," said John, looking disappointed. "How're you going to stop invaders, then?"

Sherlock dropped his hands again and gave his flatmate a look of incredulity. "John, it isn't a fortress," he said testily. "Forget the castle and go back to the palace."

"Well, I don't know. Have you got Buckingham Palace plotted in your mind or something?"

"Hardly."

John let out a sigh of frustration. "Sherlock, all you've given me to work with is a palace with towers. Bit broad, don't you think?"

Sherlock replied mildly, "That's all you've asked about so far." He shifted a bit, settling himself more comfortably on the sofa and trying not to think about how long this was likely to take, judging by the rate of productivity in their conversation thus far. Out of the corner of his eye, he watched as John struggled to come up with a better approach.

"Okay, then – tell me what it looks like on the outside. Windows, I assume?"

"Nope."

John blinked. "What d'you mean, no?"

"No. As in, no, there are not any windows."

"But –" John seemed to choke on that a bit (unsurprisingly, Sherlock thought. Conventional John wanted a conventional palace with lots of windows). "Why not?"

"No need."

"But why not?"

Sherlock closed his eyes and paused slightly before replying. "Because it can't be seen from the outside," he said finally.

John frowned. "So it's – what, invisible?"

"Don't be absurd," snapped Sherlock. "If it were invisible there would be no point, I wouldn't be able to see it in the first place."

"Okay, well, you explain, then." John folded his arms. "Because you're seriously confusing me here."

Staring at the ceiling so that he wouldn't have to see John's expression of righteous indignation, Sherlock explained.

"It can't be seen from the outside because there is no outside. There is nothing beyond the boundary, nothing exists outside of it." He glanced over to see John open his mouth in protest, but Sherlock cut across him before his flatmate could do more than sputter. "And by nothing I mean less than nothing. Nothing is still something – nothing is a void, a vacuum, but there is no void." He paused, his tone thoughtful. "I'm not sure there's even a word for it, actually," he added. He looked over at John again. "Do you understand?"

John's eyes now had a slightly wild look to them. "No," he managed to say, "but we can skip over that part, I think. Erm... how about the inside?" It was clear that he was a bit apprehensive about what might come next.

"Straight hallways. Square rooms. Ramps, not stairs. Very little clutter."

"Well, that last part just figures, doesn't it?" muttered John. "Your mind never has any clutter – you have an auto-delete button for anything you don't think is important."

Sherlock shrugged. "I have to keep things organised," he said, as though that explained everything.

"Well, I think I'm getting a better picture now," John admitted, squinting a little. "What's this whole place made out of, by the way? That would help."

"Glass," replied Sherlock, exhaling slowly.

"Glass?"

"That's what I said." A pause, then for clarification – "Purple-tinted glass, in fact."

"Purple... tinted..." John shook his head in bewilderment. "You are – unbelievable, Sherlock. Really. Only you could create an entire mental palace out of glass. Should I even ask why it's purple?"

"No."

"Okay, why's it purple, then?"

"Did you not hear my answer, John?" asked Sherlock irritably.

"My hearing's fine, thanks." Brickable smile.

Sherlock glared up at his flatmate from his place on the sofa. "Then why don't you try using it?" he suggested pointedly.

"I am," said John innocently. "I just didn't hear an answer yet."

"Oh, for God's sake – I don't know why it's purple, John, it just sort of happened, alright? Are we done with this yet?"

"What?" John leaned forward, an expression of surprise on his face now. "How can you not know why it's purple – Sherlock, you created it, remember?" He paused, then added blandly, "No pun intended or anything."

"It's not important, John."

"You never know." Sherlock could almost hear the mischievous smirk on the other man's face. "Might mean something."

"Exceedingly unlikely."

"But not impossible. Remember, Sherlock, when you've ruled out the impossible, whatever remains, however improbable, must –"

"Oh, shut up!" Sherlock seized a nearby cushion and threw it at John, who caught it, laughing openly now. The consulting detective propped himself up on one elbow, twisting so that he was half facing his flatmate. "You're enjoying this, aren't you?" he accused. "You think it's all so funny..."

"Sherlock," chortled John, "that's because it is funny. You're getting all defensive about a place that doesn't even exist."

"It's exists in here!" Sherlock retorted, stabbing a finger in the direction of his head. "And that's all that matters!"

John held up a hand. "Okay, okay," he said, still chuckling. "I'll stop getting on your –"

A half-second pause ensued in which Sherlock knew exactly what John was thinking of saying, and also hoped that the other man wouldn't say it.

"– case about it now," finished John, and he promptly buried his face in the cushion to muffle his laughter.

"Once again, I've been impaled on your rapier-like wit," said Sherlock acidly. He dropped back onto the sofa and stared up at the ceiling. "Now if you don't mind, I want to solve this case within the next hour, so if you would kindly shut up –"

John lowered the cushion far enough to ask, "How're you getting on with that, then?"

"I was fine until you interrupted me," replied Sherlock. "It's got something to do with the victim's cat obsession, I'm just not sure what, yet –"

Anything further he had planned on saying was drowned out by a loud snort of hilarity coming from John's direction. Sherlock drew in a deep breath, and spoke without turning his head.

"John, which word in the phrase 'shut up' did you not understand? I'm sure there's a dictionary lying around that you could put to good use..."

"I'm sorry, Sherlock," John gasped out, emerging once again from behind the cushion. "It's just – I got this image –" He wiped away a tear of laughter. "A purple-tinted glass palace filled with cats –"

Glancing over this time, Sherlock watched testily as John succumbed to hysterical laughter. He really didn't see what was so funny about this. After all, it wasn't as though he would ever resort to putting actual cats in his mind palace.

Arranging his features back into a semblance of inscrutability, Sherlock left John to his antics and retreated back into transparent, purple-tinted safety.

I'd very much like to hear from anyone who reads this, seeing as it's my first actually humorous Sherlock fic - reviews are appreciated more than I can say. May the Force be with you.