Another kid!fic. Based on a headcanon of mine. Because I absolutely adore brotherly love.

Can be read as a continuation of 'Big Brother'.

Enjoy.

The Science of Deduction

"No, I am not going to read to you," Mycroft stated before his brother even had the chance to open his mouth and say something. "You can read something by yourself once we're home again, if you want to. You are old enough. And now behave."

Sherlock crossed his thin arms in front of his chest, his jaw tensing. "But it's boring here!" he complained.

Mycroft shot him a scolding look. "Family meetings, Sherlock, are not meant to be entertaining. They are a simple matter of loyalty. Behave and don't act like a three-year-old."

"I am not!" Sherlock insisted vehemently. "Why do I even have to be here? Mummy and Father are talking over there, and we just have to sit here! It's so futile-"

"Sherlock," Mycroft interrupted his little brother. "Behave. There will not always be someone around to entertain you. Get used to it. And now be quiet."

Sherlock did indeed shut up, his expression an odd mixture between sour and whiny. He was silent for almost five minutes, surprisingly obeying Mycroft.

"My?" he then asked in a tiny voice that again reminded Mycroft that his brother was only six years old.

"I have told you not to call my 'My' in public," he nonetheless reprimanded his baby brother.

Sherlock looked at him pleadingly.

Mycroft sighed. "What is it now, Sherlock? What do you want now?"

His brother did not waste any more seconds. "Why does that man over there hold his head so strangely?" he asked.

Mycroft looked at the man - an uncle of their father's, though not much older than Father himself - and immediately noticed. "You can see by yourself," he told his brother instead. "Study him closely. Observe."

Sherlock's eyes were fixed on the man for a few minutes, all thoughts of boredom forgotten. "He did not sleep well?" he then suggested.

Mycroft nodded, encouraginly. "Very good. What can you see?"

It took Sherlock a few more minutes to make another guess. "He was left by his wife?"

This time, Mycroft frowned. "Not entirely. Look at him. What do you see?"

After a few more moments, Sherlock shrugged. "I don't know."

Mycroft smiled. "Then listen to me and learn." He forced his mind to focus. "He is a Holmes, nonetheless his suit is crumpled. No wife then, to iron his clothing. He does not wear a ring anymore, you can still see where it once was on his right hand. But the outline of the ring is already darker than his arms you can see because his shirt cuffs are too short. So, removed it quite a time ago - when he left his wife. That is also why he got rid of the ring - he got rid of his wife.

He is in obvious distress, as his face clearly displays, but not because of his wife. The question is - why did he, a Homes, leave his wife and want rid of her? Because he obviously had an affair. He…"

Mycroft noticed Sherlock frowning. "What is an affair?" he asked after having been listening quietly for all of Mycroft's explanations before.

Stupid.

His brother did not even understand the concept of marriage yet, how could Mycroft expect him to have a certain knowledge of affairs?

"It means that your spend more of your free time with another, mostly younger woman and neglect your wife at the same time," he explained.

Sherlock's frown deepened. "But why would he do that? If his wife was ironing his clothes for him?"

Mycroft had to fight back a chuckle. "Sherlock, there are still many things you have to learn. If a man does not consider his wife attractive anymore, he often begins an affair with a younger woman," he clarified.

"But why…?" his brother's confused voice formed another question.

Mycroft raised one eyebrow. "Do you want to hear what I am about to tell you, or don't you?"

Sherlock looked at the floor. "Yes, My. Please. I won't interrupt again."

Pleased, Mycroft continued his deduction. "So, he had an affair. His hair is dyed, but the grey roots are already showing again. So, lack of personal care. He most likely coloured his hair for his new girlfriend, but now he has stopped doing so, stubble of beard visible on his face, too. Why? Because his girlfriend quite apparently left him and threw him out of her flat. He holds his neck to stiffly since he has spent the past nights on the sofa, most likely in his wife's living-room. I can…"

"That's great, My! Can you teach me? I want to do that, too!" Sherlock exclaimed.

Mycroft only gave his brother a scolding look. "What did we just agree on, Sherlock?"

Sherlock managed to look guilty and still excited at the same time. "To not interrupt you. I am sorry."

Content and a tiny bit flattered because of Sherlock's obvious enthusiasm and his earlier comment, Mycroft went on: "I can only guess about the reasons for his affair breaking up with him, but I am fairly sure to be right about what I told you. If you were to ask him, he would confirm each of the facts."

The admiration in Sherlock's eyes made Mycroft smile.

"Can I do that, too?" his brother wanted to know. "Just like you?"

Mycroft gently laid a hand on his brother's bony shoulders. "Of course."

"Will you show me?" Sherlock asked anxiously.

Mycroft looked his brother in the eyes. "Always, Sherlock. I will show you. Look. Observe. Deduce."

He let his gaze wander through the hall. "Tell me about that woman over there."

And Sherlock tried.

o

When they were back home in the evening, Sherlock, probably already sent to bed by the nanny, appeared, as he had so many nights before, on Mycroft's doorstep.

"My?" he asked upon entering. "How do you do it?"

Mycroft did not even bother to look up from his essay. "What?"

Sherlock flopped down on Mycroft's bed. "The thing you do. How you know everything about people."

Mycroft finished his sentence. "I told you, Sherlock," he then replied. "I observe. Closely. And I draw conclusions. Oh, and please, do not call it a 'thing'. Deduction. I deduce."

When he turned around, he could see that Sherlock had already made himself comfortable in Mycroft's bed.

"You can't sleep here, Sherlock," he protested.

His little brother's head reappeared from beneath the duvet. "Why not?"

Mycroft stood up and crossed his arms. "Because you have got a room of your own. And a bed of your own. You cannot have mine all the time. Sherlock, you are too old to sleep in my bed."

Sherlock made a face. "Deduction," he then changed the topic again. "Will I be able to do it like you one day? To know everything?"

Mycroft sighed. His six-year-old brother was lying in his bed and asking him questions about deducing, after a rather frustrating day - apart from the little deduction game with Sherlock - when Mycroft originally had intended to finish his essay.

"If you try and practice, you will," he said. "Now go to your own room or I will call the nanny."

Sherlock did not even move a muscle in his face, ignoring Mycroft's order entirely. "Do you think I need a special room for it in my mind palace?" he wanted to know.

Mycroft sighed again. "Deduction, Sherlock. And yes, if you want to."

Sherlock's smile was genuine and excited. "Yes… My?"

Mycroft approached his bed, intending to remove his younger brother from it and carry him to his own room, if necessary. "Stop calling me 'My', brother," he commanded.

Once more, Sherlock was not impressed. "Should I name the room?" he wondered, Mycroft was not even sure if he was supposed to answer.

"Get up," he ordered again. "Sleep in your own bed."

"But My!" Sherlock protested, as always not paying attention to Mycroft's demand. "You're not going to bed yet, why should I not sleep here? I can leave if you want to go to bed!"

True. Not that Mycroft needed his bed right now. But then, on the other hand, his brother would grow older and older, and somewhen, he simply had to learn that he was not supposed to occupy his elder brother's bed.

"Deduction… The science of deduction, I could call it. Is it a science, My?" Sherlock interrupted his thoughts.

A science. An interesting point of view. One Mycroft himself had never had before. "Yes, I think you could call it a science. It's…"

"A science only you and I have knowledge of," Sherlock mumbled, obviously fascinated. "It will get the room right next to yours," he added, in an even more quiet voice.

Mycroft's room.

He had a room of his own in his brother's mind.

For a short moment, Mycroft wondered what might be inside it. Whether it was full of good memories, or of bad ones.

And then, out of sentiment, because of a certain sensation he knew all too well, he heard himself saying: "Fine. You can stay. But only as long as I am not intending to sleep."

As he sat down on his chair again, refocusing on his essay, and his brother once again snuggled down in bed, he scolded himself for not being strict enough.

His brother always managed to hit his soft spot for him. Always. And Mycroft could never bring himself to do what was right.

Sentiment.

Maybe it was time to move Sherlock's room in his own mind out of the centre.

Somewhen. But not yet, Mycroft decided as soon as he noticed that his baby brother had fallen asleep, quickly and peacefully, unfortunately once more in the wrong bed. Well, he simply would have to carry him to his own room later in the night.

But not yet.

Thank you for reading. Please let me know what you think.