"But don't worry, that's not a secret." Sheldon pointed at Leonard woozily, leaning on an end table for support. "Everybody knows."

As usual Leonard ignored this jab. It was often difficult to know whether Sheldon was being deliberately insulting or not; it was just as likely that he had no idea that what he was saying could be hurtful. Leonard wasn't in the mood to try and figure out which was the case. He was just glad that he finally knew what the deal was with Penny, and that it wasn't his fault at all! It made him feel a little better to know that she had her own insecurities, but he didn't want her to feel bad about her lack of education. He'd have to figure out what to say to her...

A crash yanked him off that train of thought. Leonard got up and peered down the hallway: the bathroom light was on and the door was cracked open. Further investigation revealed Sheldon on the floor, tangled hopelessly in the shower curtain that he must have grabbed onto for balance before ripping it down.

"Are you okay?"

"Help! It's a trap!"

Sheldon flailed uselessly as Leonard disentangled the plastic from around him and threw the whole mess into the shower. He'd fix it in the morning. Sheldon looked up at him from the floor, uninjured, giggling softly to himself. "Everything is all spinny..."

"Spinny, huh?" Leonard got a grip on Sheldon's arm and hauled him up. "Is that the technical term?"

He deposited Sheldon in his spot and went to make a pot of herbal tea. He'd have to figure out the situation with Penny in the morning. It was very late but he needed to get Sheldon down for the night before he went to bed himself, or else who knew what he'd find when the alarm went off? Howard's suggestion of crating him overnight didn't sound so bad, actually...

Sheldon slithered off the sofa and flopped dramatically onto the floor.

Leonard wondered if there was any Benadryl in the medicine cabinet, and whether or not it would interact with the Valium that Howard had already given him... and whether or not he would really care if it did...

"Leonard! Leonard, help!"

Leonard looked away from his tea-making. Sheldon was up, hugging his whiteboard stand desperately.

"What the hell are you doing?"

"My whiteboard is escaping! Don't let it get away!"

Sighing, Leonard stomped over and pried Sheldon off the whiteboard. "Here, sit down right here." Leonard pointed at the floor. He pulled the board off the stand and propped it up against the back of the coffee table facing Sheldon, then handed him a whole box of markers. Some nice calculus would keep him occupied until he wore himself out and fell asleep.

Leonard settled at the kitchen's island with his tea, prepared to keep watch over Sheldon's bleary mathematical progress until one or both of them passed out. It was already past two and Leonard had no intention of getting up at six and driving them to work. Call it a sick day, call it a Sheldon's Nuts day, call it whatever. Well, that wasn't really fair. Sheldon had always been very open about his inability to keep secrets and Penny shouldn't have put him in such a position in the first place. Not that it was Penny's fault, either. Maybe he could bolster her self esteem by pointing out that her social skills far surpassed his and Sheldon's—

"Why don't we have any white markers?"

Balls, and here he was hoping the time would pass quietly.

"If you wrote on a whiteboard with white markers you wouldn't be able to see it, doofus."

"Then what will I do with the white numbers?"

It wasn't the weirdness of the question as much as the genuine distress in Sheldon's voice that made Leonard seriously contemplate what they might be able to use as a white marker. There was a white-out pen in Leonard's desk (not in Sheldon's of course, because if Sheldon misspelled a word he'd write the whole page over) so Leonard got up and fetched it for him. As he crouched down to hand it over, much to Sheldon's delight, something odd struck him.

The equation was familiar, something to do with the speed of light traveling past a black hole in the vacuum of space. Sheldon's handwriting was perfect as always, but each number was in a different color. All the sevens were orange, all the nines were red, all the threes were black. And each level of the equation had a few gaps, which Sheldon was now filling in with barely visible white zeroes. Leonard was almost tempted to ask why, but it was late and he already knew the answer would be nonsensical and, well, Sheldony and he didn't think he could take it. Color coded math wasn't that weird, anyway.

But as luck would have it, Sheldon was done with this equation so he erased it almost as soon as he was done filling in the zeroes. He wrote another one at the top of the board, taking time to write each number in a different color—and interestingly, in the same colors they'd been before. The sevens were still orange, the threes were still black.

And that in and of itself still wasn't that weird. Sheldon denied being obsessive-compulsive but his whole personality said something different, so color coded numbers fit right in with the breakfast cereals organized by fiber content and the labels on all their ink pens. It was just that there was some nagging pattern to it, and Leonard couldn't quite see the big picture. This was important, but why?

Sheldon was humming quietly to himself, paying no mind to Leonard hovering over his shoulder. Leonard gripped his mug of tea in quiet anticipation... anticipation of what? Good grief, he was suddenly on edge as if he were about to witness some paradigm-altering event. It was just Sheldon doing math for fun, which wasn't unusual at all. Sheldon's office at work had multiple whiteboards, all with math on them. Math was an everyday part of their lives.

...come to think of it, Leonard recalled seeing this colored numbers phenomenon a few times before, always in Sheldon's office. He hadn't paid much attention to it, but now wondered if it were some kind of system Sheldon only used in private when he thought no one was watching.

Maybe Leonard ought not to be watching now.

Ridiculous. It was just math, and it wasn't like Sheldon was telling him to go away. It was the lateness of the hour that was getting to him, Leonard decided. Sheldon was working the problem now and everything was fine, if colorful.

Except that it wasn't, because Sheldon wasn't working the problem. He picked up a marker and, starting at the right side one line under his equation, filled in a purple four. A green one went on the far left, and two red nines and a white zero filled in the gap. He filled in the next few steps the same way, by color rather than by calculating anything. He showed no hesitation, he wasn't doing the math, he was doing the colors.

But as far as Leonard could tell, by dividing it up in his own head, the answer was right.

Or, he realized, from Sheldon's point of view the colors were right.

This must be it, then. The one big piece of the puzzle that everybody had been missing for years. The one idiosyncrasy that could explain Sheldon's whole life. This was what gave him his edge, got him into college when he was eleven and made the board of directors think he had such a beautiful mind—and those judgemental old men didn't even know the half of it. Leonard couldn't prove it, but there was one way to find out and it was lucky that Sheldon was already drugged.

"Hey, Sheldon?"

"Mmmmm?" Sheldon bobbled his head curiously in Leonard's direction. In doing so he dropped his marker, which rolled away under the armchair. "No, marker! Don't leave me!" He scrabbled under the chair for it in a panic.

Leonard went to pour more tea.

Sheldon squeezed himself out from underneath the chair triumphantly, hair sticking up in every direction. He then knocked over the whiteboard and promptly fell over himself in trying to rescue it. Ultimately he resigned himself to simply cuddling the whiteboard and letting the marker roll away to be with its marker comrades scattered on the area rug beside them.

Satisfied that Sheldon was settled down for a moment, Leonard sat in the armchair and propped his tea on his lap.

"So, buddy," he began as nonchalantly as possible, "what's your favorite number?"

"One," Sheldon murmured, rolling so that he was lying on top of the board. Probably to keep it from running away or something.

"Why one?"

"It flies solo. And it's green. I like green."

"Oh yeah? What about seven?"

"Oh no, I don't care for seven. Seven is kind of a bitch, that's what's wrong with eight."

"What's wrong with eight?"

"It's traumatized from being bullied by seven all the time, of course. They spend a lot of time next to each other."

Sheldon continued to roll around on the board weirdly for a few minutes, drawing invisible math in the air with one finger.

In addition to shock at Sheldon's discovery of profanity, Leonard was somewhat humbled by the evidence he'd just uncovered: Sheldon had synesthesia, and that explained—well, not everything, but definitely a lot. Leonard's mother was a world-renowned psychiatrist and when he had been in high school she'd written a lot on the disorder—although Leonard would argue that perhaps it wasn't a disorder as much as it was just a rare place on the spectrum of human evolution.

The kids in his mom's studies had reported seeing music as wavy blocks of color passing before their eyes, weird shapes interacting with the real world and sometimes hard to distinguish from it. Some of them could hear colors, and a few could smell sounds. How much more interesting the world must be for them, being able to sense everyday things so differently from everyone else.

And yet—he eyed Sheldon, who was now trying to follow the ceiling fan with his head—how much more cluttered and disorienting the world must be for them as well. Every sound that appeared in their vision must interact with every other sound, creating rainbows and distracting arrays of color, muddling their ability to focus. For most it blended seamlessly with their other senses, for a few it made life difficult.

Leonard sipped his tea quietly, contemplating this new way of understanding his roommate. It wasn't just the numbers, probably. Many synesthetes reported more than one form, and it was well known that the phenomenon could affect a person's behavior dramatically. Maybe Sheldon smelled music and heard shapes, too. A heavy dose of synesthesia on top of the OCD and how he struggled with social interactions might complete a trifecta of suck that interfered with Sheldon's ability to cope in the real world. Perhaps he spent a lot of time in a state of sensory overload and that was what made him the way he was: bat crap crazy.

"Hey Sheldon, what color is the Star Trek theme?"

"Mostly blue and purple. And elliptical. Mmmm." he put his hand out in front of his face and drew an invisible ellipse. "Like the orbit of a planet. It has two foci. That's a fun word. Foci, foci, foci."

"And why can't we listen to music in the car?"

"We might run over a b-flat. They're kind of jumpy. We don't want to commit musical homicide." Sheldon giggled. "Foci, foci..."

"Do you want some tea?"

Sheldon thought about this. "Is it going to taste like Wollowitz's milk?"

"No, it's just regular tea, I promise."

"Okay. Can it have honey in it?"

"Sure, but you have to sit up and drink it, so go get in your spot."

Sheldon wobbled up onto his hands and knees and made for the couch while Leonard poured the tea and located the last of the honey. As an afterthought he grabbed a kitchen towel and handed it over to Sheldon with the tea, in case he spilled, so maybe they could avoid a meltdown.

Sheldon drank his tea in relative quiet while Leonard picked up the board and markers. After a moment's reflection, he erased it while Sheldon was intensely studying the table lamp beside him. He put the white-out pen back in the desk drawer and turned out the living room lights. Sheldon's eyes were more glazed than they had been a half hour ago, so maybe he would now be able to fall asleep in his own bed.

"Come on, buddy, let's hit the hay."

"Hay hay hay" said Sheldon, steadying himself on walls and furniture as he followed behind. "Buddy bud bud..." They entered Sheldon's room and Leonard turned off the alarm clock while his roommate climbed into bed. They were so not getting up in the morning.

"Sheldon, where are your shoes?"

"Ummm." Sheldon looked down at his socked feet as he sat on the edge of the bed. "I had them at Koothrapali's. Maybe."

"Never mind, we'll find them in the morning. Lay down."

Leonard pulled the cover up over Sheldon and surveyed the room before he turned out the light; there was nothing out of place lying about that he might get up and hurt himself on. He stood there in the dark hallway for a long minute before he shut the door, and another long minute afterward. Sheldon stayed put, so Leonard headed for the kitchen. The Roommate Agreement said that no dishes could be left unwashed overnight.

As he dried the mugs and teapot and put them away, Leonard wondered how much more tolerant the others would be of Sheldon's behavior if they knew. This was why they couldn't whistle, or listen to the radio in the car, or go out dancing at a club, or play musical instruments when Sheldon was home. This was probably why he hated birds. Hell, maybe it was why they had to have oatmeal on Mondays, because Monday doesn't smell like fried eggs. Maybe they went to the comic book store on Wednesday because Wednesday was red and so was The Flash. But even as the thought crossed his mind he could already hear Penny singing loudly and Howard repeating "What color's my voice? What color's my voice?" and he knew why Sheldon had never told anyone.

Hopefully in the morning Sheldon would have forgotten all about his drug-addled confessions and they could go on as if Leonard had no idea. They'd have a late breakfast and probably deep clean the apartment, Sheldon's usual ritual penitence for missing work. Synesthesia never needed to come up again. And the more he thought about it, as he tucked his own covers in around himself, the more he knew that he could never tell a soul. Like, ever. Maybe Sheldon couldn't keep other people's secrets because he had enough of his own secrets to contend with. What Leonard wouldn't give, though, to spend a day in Sheldon's head and to see the world as he saw it.

But nobody would want to spend that much time in Sheldon Cooper's head—not even Sheldon himself.