A/N: I know I should be working on the next chapter of my kid!Sherlock story, but, as a celebration of the end of finals, my friends and I watched Fight Club, which I'd never seen. I did figure it out the moment Tyler walked onto the screen, causing my friend Lydia, who had never seen it either, great consternation as to what I could possibly know and how I could possibly know it... and thus a plot bunny was born…and I've finally decided to post it. I'll probably make a little drabble series for things like this that pop into my head.

"John, all I asked you to do was hit me, and I really don't understand-"

John stopped laughing long enough to say, "The first rule of fight club is never talk about fight club? Really, Sherlock, you've never heard that before?"

"I have no idea what you're going on about—I need you to hit me for an experiment. Here." Sherlock pointed at his cheekbone. "I need to monitor the progression of bruising-"

"Sherlock, I'm not going to hit you in the face."

"Come on, please. John. I'll get the milk if you do it."

John sighed. His flatmate looked like a sulking fourteen-year-old. He weighed the absurdity of punching Sherlock in the face and letting him walk around London looking like a…God, John didn't even know. Sherlock could bloody well act like bloody Tyler Durden sometimes, and even though John didn't buy the sociopath he certainly bought the boredom.

"No," John said finally, "I'll hit you, if it'll make you happy, but don't get the milk, I've already done the shop and we've got milk, tea, orange juice- besides, you'll probably get the wrong kind and slip something into it or blow it up somehow on the way back to the flat, or throw it at a murderer's head or something—no. I'll hit you if you'll watch a film with me after."

"No need to go to all that trouble to ask for a date, John," said Sherlock, smirking to hide the fact that he was internally grinning like a child at getting his own way.

"Shut up," said John.

"No, really, I can tell you fancy me, and it's a bit bloody pathetic to be honest-"

John punched Sherlock in the face. Hard.

Sherlock grinned even wider, holding his cheek. "Thank you." He pulled a digital camera out of his pocket and went over to the mirror to take some photos of the spot where John's fist had connected with his face.

"I'll get the milk and the DVD, then," said John, leaving his flatmate to whatever it was he was doing.

That evening, John pulled Sherlock out of the kitchen with the promise to help him photograph his developing bruise every hour "until reasonable human beings are in bed, mind," and sat him down on the sofa.

Sherlock fidgeted through the beginning of the film, once asking John,

"Is he ever going to stop whinging and do something?"

"Ssh. Yes. Wait."

"Fine." Sherlock probed his bruise with his fingers.

"Stop playing with it."

"No."

"You'll sod up the results."

Sherlock's hand went down to the sofa with remarkable speed.

"Watch this, now, it's important," said John as Edward Norton's character sat down on what appeared to Sherlock to be just another of his ridiculous aeroplane trips.

Then another man was sitting next to him. John leaned back and grinned, sure that Tyler would hold Sherlock's attention.

"He's not real, is he?" Sherlock asked instantly. "He's a hallucination."

"What?" John blurted. "How can you possibly know that?"

"Easy. When he sat down, the window seat was empty. Then the other one appeared when he looked up. Since this is a film, it could be some kind of magical realism, but this seems realistic so far, and he's been going on and on about his insomnia and how it blurs the lines of reality, so, inference, hallucination due to lack of sleep and to help him deal with his empty, boring life."

John sat and sputtered for a moment.

"So, am I right?" Sherlock asked. "I'll sit and watch anyway if I am."

"Yes," John growled, "you are."

"Are you not meant to realize that?"

"No, not really, at all, no."

"Mmhm, it's a twist. I should tell you, no one watches films with me, I'm apparently rubbish, especially at ones with twists."

"Sherlock, no one watches films with you because you haven't any friends."

"Actually, no, Mycroft watches films with me, or would…"

John gave up in disgust and sat back to watch the movie, trying not to imagine the horror of Sherlock and Mycroft watching a film together.

"You loathe your brother," said John, trying to rid himself of the terrifying idea of the Holmes brothers seated in front of a television, competing to figure out every last detail of the film within the first ten minutes.

"Yes, which is why the only time we interact with any kind of remote civility is when we're competing."

John had a thought. "I suppose the two of you dreamed up grand pretend games as children, didn't you? Tyrants and thieves and knights and all that?"

"Mm, no knights. Spies. Murderers. We'd set each other puzzles, too. Once I broke into his room and stole all the essays he'd written for university over Christmas—left him codes to find codes to find codes to find a clue to where I'd hidden them…"

"What'd he do to you?" John was slightly afraid to hear the answer.

Sherlock grinned. "Actually, he was delighted at the time. Years later he took to hiding my…" he trailed off, glancing at John's face. "Never mind."

"What? Sherlock?"

"Nothing."

"Sherlock, are there drugs in this flat?"

"Yes. Acetominophen, aspirin, nicotine, caffeine-"

"You're a prick."

"I'm precise."

"Pedant."

"Shut up, John, you're interrupting the film, and they're talking about explosives. Is his other personality a bomber? That's brilliant."

"No, Sherlock, it's not."

"You can actually do that, you know, you take three parts gasoline-"

"I don't want to know, and I don't want to move into an abandoned shack in the middle of nowhere and watch you start a terrorist organization so please, please, for the love of anything sane, do not blow up the flat."

Sherlock crossed his arms and drew his knees up to his chest, assuming the defiant pout of a thwarted toddler. "Fine."

John settled back to watch the film, content for a few moments.

Then he noticed that Sherlock had his phone out.

"Sherlock, who are you texting?"

"Whom."

"Sod off. Who?"

"Mycroft."

"Why?"

"I think it might be nice if I stopped off and visited his house while he's in Geneva and I don't want him to be able to track-"

"Sherlock, NO."

...

The next morning, John was surprised to find the flat quiet when he awoke, with no evidence of nocturnal violin playing or dangerous experiments with alkaloids in the Marmite. He glanced again into the living room, and was shocked to find his flatmate curled in a chair, giving every appearance of being asleep. All was peaceful until John opened the freezer, hoping to thaw a bit of orange juice to have with his toast.

No, there was no severed head in the freezer.

There was also no orange juice. John ran a hand through his cropped hair. He knew he had bought it- a whole litre of frozen concentrate, since Sarah had mentioned wanting to try a smoothie recipe or something she'd seen on telly and he thought he'd be the domestic hero and have a bit on hand (also hoping, of course, that she'd be hanging about the flat to ask).

Did you know if you mix equal parts gasoline and frozen orange juice concentrate, you can make napalm?

John glanced at the freezer. Back at the sleeping Sherlock. Back at the freezer. Was that a nip of gasoline he smelled?

"SHERLOCK!"

Fin.