"Santa's not real."
Mycroft looked up over the pages of his textbook, eyeing his little brother curiously. "Of course he is." He said naturally, as anyone would to a child like Sherlock. Well, maybe not 'like Sherlock', but any child. This was supposed to be the age where children embraced Santa with open arms, finally understanding the lore behind the presents under the tree every Christmas.
Sherlock crossed his arms, which took a bit of wiggling, seeing as he had a wooden sword in one hand. In fact, he was in his pirate garb, including a blue bandanna making a feeble attempt to hold back his wild black curls, a white button-up collared shirt that was too large for him (it had been Mycroft's, once), and poofy brown pants that billowed out before scrunching back in with elastic around mid-calf.
"No he isn't." Sherlock announced stubbornly.
"Whatever you say. Just don't tell father and mother or they'll think I told you," Mycroft sighed, going back to his homework.
Sherlock gave a triumphant little shouting noise (that made his older brother jump), raising his sword in the air. "So I was right!" he bellowed victoriously.
"What?" Mycroft looked up suddenly, realizing his mistake. "I didn't-"
"You said mum and dad would know you told me, and that implies that it is something you already know but I'm not supposed to. Therefore, Santa's not real!" Sherlock laughed at his success.
The older boy scowled at him and his trickery, annoyed that he'd fallen for a simple trap such as that. "Stop using big boy words, you probably don't even know what they mean," He humphed, his annoyance turning into a jab at his sibling.
"Of course I know what they mean, I read just as much as you do!" The boy cried.
"Just because you can read doesn't mean you can comprehend," Mycroft sighed. "Oops, there's another big word. It means 'understand', since you didn't know."
"I know what comprehend means," Sherlock stuck out his bottom lip, considering hitting his brother with his sword. However, Mycroft had a considerable number of years, pounds and inches on the young Sherlock, and he calculated that the chances of a clean get-away were quite low indeed. "Do you want to know how I knew?"
"I don't know, you read it somewhere?" Mycroft sighed, wishing his brother would choose someone else to bother.
"No, about Santa."
"Oh, how riveting," Mycroft was dripping with sarcasm. "How I'd love to hear how you blew a hole in the theory of the wondrous Saint Nick."
Sherlock either ignored or didn't notice the sarcasm in his brother's tone, and he began pacing back and fourth, swinging the wooden blade forward and back in his grasp as he walked. "It's simple, really. For him to fly all the way around the world in one night is the first giveaway, unless he traveled through the timezones in the proper direction so that there was less time lost, at an almost impossible speed, that is. Then there's the time it takes to stop inside every single household to both deliver presents and eat whatever snack has been left out for him. That time would make any sort of progress nearly impossible by any standards, he would waste the entire night just trying to get through one timezone.
"Then, there's the matter that he only visits homes that celebrate Christmas. Though that does reduce the number of homes to visit, it disproves the fact that he delivers presents to every boy and girl. Henry Jooley down the street celebrates Hanukah and he's never gotten a present from Santa."
Mycroft was getting thoroughly peeved at his brother's ramblings, and scrunched his face up in annoyance as he tried to read the words on the page before him. He kept trying to drown out the ramblings, but it wasn't quite working and he couldn't very well focus on both at once.
"Then there's the Reindeer. Putting aside the fact that Santa's can supposedly fly, I looked up what the weather's like at the north pole, and it is much colder there than it is where they naturally live. Santa couldn't keep the reindeer there, they'd freeze. They wouldn't even be there in the first place for him to use to pull his sleigh, do you see?"
"Yes, yes, are you quite done?" The older boy huffed.
"Almost," Said Sherlock, his little voice building as his deductions grew (much to Mycroft's chagrin). "The presents themselves are a dead giveaway. Every year we receive lavish gifts from mum and dad, and yet our tutor's son gets a worn box that was obviously obtained at discount or resale. Santa would not differentiate children between richer or poorer, so the presents have to be from the parents!" He ended by shooting his sword-arm up in the air in triumph.
Mycroft sighed, closing his textbook around his finger (to hold his place). He leaned forward, examining his little brother. "Very good." He cooed sarcastically. "Now how about you go bother the housekeeper, and leave me alone?"
Sherlock pouted, kicking his feet. "I thought you'd be impressed
" He said quietly.
Mycroft rolled his eyes. "Honestly? I'm shocked you even fell for it for this long. The entire story is laced with impossibilities." He leaned back into the chair, re-opening his text. "And when you get rid of the impossible things, the truth is whatever's left."
Sherlock pondered this. "What if what's left isn't the probable solution?"
The other boy's grey irises flicked up to gaze at the child out of the tops of his eyes. "Do you even know what those words mean?"
"Of course I do."
"If it's whatever's left, it's the truth." Mycroft repeated, going back to his homework.
Sherlock fit this together in his developing mind. "When you've eliminated the impossible, whatever's left, however improbable, must be the truth." He said slowly, constructing the single thesis from the different thoughts.
"Hm? Yeah. That's what I just said." Mycroft said vaguely, not even bothering to look up this time.
Sherlock, armed with this new theory, decided to finally let Mycroft have a bit of peace as he wandered off to the library to expand his growing knowledge.