Just an idea that popped into my head, and I had to write it down! Review and tell me what you think and I may continue with it, because i like the idea! :)

John was following Sherlock, (as usual) through a park. Sherlock was darting around to and fro, trying to find clues associated with their latest case. John was getting increasingly more weary and he wanted to go home but he just knew Sherlock wouldn't be giving up on this 'possibility of clues' any time soon, and he couldn't exactly just leave him to it, not with what happened last time...

They had been chasing a man down the streets of London and John had managed to lose track of Sherlock. He was trying not to panic as he walked back home, knowing that he would be informed of everything later. When he was, he discovered that Sherlock had managed to commandeer a taxi cab, (despite being too young to drive, let alone drive a taxi!) and driven it manically around until he managed to "skillfully" (Sherlock's description) run the man over, with the skillful part being that it only knocked the man out. This was NOT going to be happening again John thought, as he kept his eye on Sherlock as he moved away.

They ended up in the children's play area in the park, by the slides and swing and immediately obtained disapproving, overprotective looks from the mothers, who kept a firm eye on their own children and the pair. John sighed, he just didn't understand their prejudices, the mothers just saw a pair of teenagers and were automatically wary for their children's safety, without even knowing them!
Although to be fair to them, Sherlock did look rather peculiar, he is tall for his age anyway, and currently wearing a dramatic long black coat with his dark curls falling around his head like a wild disheveled birds nest. What made it worse was he was on the floor on his front looking at a patch of grass by the edge of the swings through his magnifying glass, and John was sure that in the mothers minds this was Sherlock 'ready to pounce' on their unsuspecting children.

John was looking around standing by the swings whilst waiting for Sherlock to finish his deductions so they could move on, when a man came to stand by him. The man had just picked up his small child who John reckoned looked around 5 months old. He was gently rocking the child, whose face was resting on his shoulder. John looked over and smiled at the child who smiled back. Then he put his hands over his eyes and quickly moved them away, playing peek-a-boo. The man saw John out of the corner of his eye, and turned to talk to him.

"Hello" The man said,

"Hello" John said smiling, wary of whether or not it had been appropriate to smile at someone else's child. "I'm sorry, your baby is very cute!" He said.

"Oh, he can be a rascal though!" The man smiled back, reassuring John.

They continued to talk together, until a worried look came over the man's face. The look changed to sheer panic and then determination. John, concerned looked to where the man was looking, and spotted a small figure darting between some trees a little way off. The man whipped out of his pocket a badge, which he showed to John.

"Secret Service, I'm going to have to ask you to hold my child. This is an emergency and a matter of utmost importance."

John looking panicked, was handed the child from the man, and took a step backwards to stand next to Sherlock who had risen from the floor on hearing what the man had said. Together they watched him run towards the cluster of trees.
It all happened so quickly, they barely had time to realise what was happening. As soon as the man disappeared behind a tree they heard three loud gunshots which rang through the air and echoed away. A small figure, although not the man they had been talking to, promptly ran away from the trees.
Sherlock in this situation would have usually gone running after the shooter, regardless of the danger. But this was different.
He turned to John who sucked in a breath, his eyes glazed in shock. He looked down at the child in John's arms, who was looking up at the two with wide eyes and a small mouth curved into small 'o' shape. John gritted his teeth, feeling dizzy and overwhelmed.
How swiftly everything had changed. He almost didn't believe what had just happened. He looked around at the park but there was nobody there anymore. John had a feeling that they must have left before the shooting or else they would have heard some screams from the overprotective mothers, whose overprotectiveness didn't seems so ridiculous to John anymore.

"What, are we going to do about the child?" John whispered to Sherlock, not trusting his voice to get any louder.

"I'm checking the man. Stay here"

"Sher- Oh for-" John stammered as Sherlock bolted towards the trees.
He tried to rock the child in his arms the way he had seen mothers do, he patted him on the back and stroked his hair, murmuring that everything was going to be okay, although he was reassuring himself more than the child, who was too young to understand what had just happened.

As Sherlock came back, he shook his head at John's hopeful expression.

"No. He's- he's dead."

"Oh, god. What are we going to do about the child then? Sherlock?"

"We ring Mycroft. Come on."

Sherlock started striding towards the park's exit and towards his flat, John followed behind holding the baby. Sherlock dialed in a number as they walked.

"Mycroft. We have a...situation."

Despite Mycroft only being 10 years older than Sherlock, that making him only 26, he already held a high ranking position of authority in the British Government.
He was constantly helping Sherlock to get out of trouble, and seemed to always have someone who he could bribe to get needed information. This, not even starting to use the Holmes fortune, to which there was always access to as well.

"We need you to get information... No its not for The Barmount's Case... Yes I know it was simple... Oh course i've solved it." He snapped.
"This is something different. A man, has left us... well in charge of a child." Sherlock looked around at John holding the baby. "Yes Mycroft. A CHILD. Yes we have him here... He's... 4 months old... Dark hair... Slightly wavy... Blue eyes...No i'm not describing myself... Yes, he does look like me... No Mycroft, i've not cloned myself!... No we did not take him! We were given him!... In a park... Yes, a man... He was Secret Service... fine... yes... no. We will be at the flat." He hung up and put away his phone, and then got out his keys as they approached the flat door.

This flat was a apparent "Necessity" of Sherlock's, bought 3 years ago, (with the Holmes's fortune) and it was "needed" for Sherlock to live when he was sick of the huge family mansion, and home from school. Inadvertently he usually lived there for the entire holiday. His parents didn't seems to care where he was, maybe they thought it boosted his independence, and so long as he was there for Christmas, Birthday's, and other festive occasions, they turned a blind eye to his absence.

Once in the flat John sat down on his chair, with the baby on his lap. Yes, John had a chair, this was because of the simple fact that in the holidays, he also spent the majority of his spare time around at Sherlock's. Usually trying to stop him murdering himself, or others during his states of boredom. Or helping him track down other such murderers doing the same.
Also because Sherlock would have lived quite happily in a flat only containing test tubes and tea, and would have withered away his cases seated on the floor, oblivious to the lack of furniture, and John thought that a least someone in the flat had to take control.
Sherlock started pacing the width of the room, John's eyes followed him back and forth and so did the baby's. The baby kept quiet only turning his face to John's as if to ask what Sherlock was doing. John gave him a look back as if to say 'I have no idea what the crazy guy is doing.'
The baby smiled up at John, who covered his face and then moved his hands away quickly, playing peek-a-boo. The baby loved this and gurgled and clapped his hands, so John did it again, with the same reaction. Sherlock stopped pacing and looked over.

"What are you doing?"

"Playing Peek-a-boo, he loves it!"

"That is a completely illogical game. Even the baby isn't so stupid to not realise that you haven't disappeared."

"No. But he likes it anyway... Peek-a-boo!"

"Absurd." Sherlock exclaimed sitting down in his chair and staring at the child across from him.

After a while, the baby became bored with the game and started to cry. Sherlock made a look of horror.

"What does it want? Stop it John!"

"I don't know! um, baby? What do you want?" Asked John looking flustered.

"He can't speak John, of course he can't speak!"

The wailing got louder.

"Um, okay. yes, um? Food! Maybe he wants food!"

John quickly got up placing the baby on Sherlock's lap (much to his annoyance,) and hurried to the kitchen and started to heat up some milk and found an empty water bottle which he washed thoroughly before putting in the milk.
Suddenly thinking of what Sherlock might be doing, as it was very quiet, he hurried into the living room with the bottle.
He saw a strange sight. Sherlock, who should have been holding the baby, instead was looking down in fascination at the child-who was on the floor.

"John Look!" He said gleefully.

"What? Sherlock you were meant to be holding him!"

"John, he's crawling."

"I can see that."

"He's not meant to."

"Be crawling?"

"No!"

"Why not?'

"It's obvious John. This child must be a genius."

"What?"

"They aren't meant to crawl until 6 months at the earliest, even that is rare, but at 4 months he is crawling already!"

"How do you know he is 4 months he could just be-"

"He's four months." Said Sherlock crouching down to eye level with the child and peering into the boy's eyes, who stared back.

"Okay, hang on. You don't know the basics of the solar system and yet why have you memorised the stages of development in a child?" John asked incredulous.

"It is useful. And I just know."

"Of course you do."

"Yes." Sherlock said, abruptly turning around to John. "We are keeping him."

"What?" John exclaimed disbelieving "Sherlock, what do you mean? We are 16! What do you mean we are keeping him?!"

"We can manage."

"No we can't! What do you mean? We have school!"

"We can take him to school."

"No we can't!"

"We can! We can hide him in our dorm room." He said, now determined to get his own way.

"Sherlock." John sighed, "Look, I don't quite understand why you want to keep him, but, but, well- we just can't."

"We can! I want to keep it because it has genius potential and how often does a child show that! I know what will happen! Mycroft will take it away, and give it to some mundane, stupid family who will just fill its head with all sorts of nonsense, and it will turn out like every other idiot on this planet!"

"Sherlock, it's- he's a person! You can barely take care of yourself let alone another person!"

"That's why I need you to help me!" Sherlock shouted, just as the doorbell rang-interrupting their argument.
John ran down and let Mycroft in.

Mycroft walked up the stairs, his shoes making sharp tapping sounds. His dark suit was impeccable as usual, and his hair was styled into his customary precise and faultless look. As he entered the room, he spotted Sherlock lying on the floor with the baby, and he looked down in distain.

"Oh dear. I should have arrived earlier."

"Mycroft. I'm keeping him." he said indignantly without looking up at his brother.

Mycroft breathed out a long sigh.

"Yes. I definitely should have arrived earlier."