When John came home from work to find Sherlock using his laptop he wasn't particularly surprised. Sherlock was always using his laptop, usually because Sherlock's own was too far away to be reached without leaving the comfort of his armchair (John didn't understand why Sherlock always ended up leaving his own laptop on the other side of the room – maybe it was just to annoy John?).

On this day, however, something seemed different. John couldn't say what, but there was something. So he went over to have a look at the screen and see what his eccentric flatmate was up to.

Sherlock turned the screen away from him.

John hesitated, then tried again. Sherlock turned away. John frowned. The detective obviously wanted to keep this to himself. So John went off and got changed and made a cup of tea and opened his mail. He was quite content minding his own business and reading the newspaper when Sherlock gave a frustrated growl. John looked up to see Sherlock hunched over the laptop, scowling and obviously concentrating hard.

"Everything alright?"

"Fine." Sherlock snapped.

John shrugged and went back to his paper.

Sherlock wasn't fine. He was furious. He was one of the cleverest people in Britain (the cleverest if you asked him). He was the world's first and only consulting detective. He was a genius.

And he had been defeated.

He was staring, flummoxed, at John's laptop screen where one word was blinking innocently.

Password:

Sherlock Holmes was completely and utterly stumped.

At first, John's passwords had been easy to guess. Child's play. Once John had found out about Sherlock accessing his computer he had started getting more creative (Sherlock had found some of his attempts highly amusing) but even so, none of them had taken longer than a few minutes to guess.

Sherlock had been trying to crack this one for four hours.

It was unbelievable. And infuriating and impossible and unthinkable and inconceivable.

But there it was. The proof was staring him in the face.

Password incorrect.

John Watson had beaten him.

After half an hour of Sherlock's moaning and groaning and sighing and huffing and slamming his fists down and muttering under his breath...John cracked.

"Sherlock! What the hell are you doing?"

Sherlock's head snapped up and he stared at John. There was something strange in his gaze, something desperate and disbelieving and...confused?

"Sherlock." John said, gently this time. "Is everything OK?"

Sherlock sighed, looked back down at the computer and mumbled something.

"Pardon?"

More mumbling.

"What? Sherlock, I can't hear you."

"For God's sake! I can't guess your password. OK? Happy now? I'm stuck. The great Sherlock Holmes is stuck! I've been outwitted by a bloody laptop!"

John's jaw dropped.

His stared at his best friend, trying to take in this extraordinary information.

"Are you serious? That's what your bad mood is about? My password? Bloody hell, Sherlock, just use your own laptop."

"I can't! I have to do this! It's a challenge."

John gaped at him for a moment longer, then his face transformed into a smirk.

"OK. Sure. Go for it. There's a password hint on there, by the way. If you want it."

Sherlock scoffed. "Of course not. It may be taking slightly longer than usual but I don't need a hint. How ridiculous."

John shrugged. "Suit yourself." Then he left. Sherlock heard him head upstairs to his room. He turned back to the screen. There, at the bottom of the page, were those two misleadingly innocent words.

Password hint?

Sherlock growled.

It took ten minutes for him to cave.

Your password hint: this caring lark

Sherlock stared at the screen. What?

John was delighted. It was two days later and Sherlock still hadn't guessed his password. Obviously, Sherlock could probably find a way to hack into his computer without the password but John knew he wouldn't. Sherlock was determined to win.

John knew Sherlock had looked at the hint because he'd asked him what the hell it meant. John had smiled at him.

"It pretty much means what it says. The password is something I care about."

Personally, John thought this was good for Sherlock. It was only natural that he get stuck every once in a while. And watching Sherlock's frustration rise every day was worth being unable to use his laptop because Sherlock constantly had it.

Although he was going to have to come up with one hell of a password to replace this one when Sherlock finally cracked it.

The password is something I care about.

Sherlock couldn't get this phrase out of his head. It was floating around his mind, popping up at the most inconvenient times.

The password is something I care about.

Caring. What did Sherlock know about caring? How was he supposed to guess this? It was to do with...emotions.

He had tried everything he could think of. He had even asked people what John cared about. Mrs Hudson, Harry, Lestrade and even Mycroft. They had all smiled (well, Mycroft smirked) and said he would work it out eventually.

Eventually turned out to be a week after he had originally tried the guess the elusive password. He and John were in a cab, heading back to Baker Street after the successful conclusion of a case. But John was staring out of the window, ignoring him. His jaw was clenched and Sherlock knew he was angry. He didn't understand why. He had known exactly what he was doing when he goaded the criminal into firing the gun at him. Evidently, John didn't understand.

Twenty minutes and a lot of arguing later, Sherlock understood the theory behind why John was angry. But he didn't fully get it until John said;

"How would you feel, Sherlock, if I irritated a dangerous criminal into shooting at me?"

Oh.

Finally, Sherlock looked at his friend. "Why do you do this, John? Put up with me, and look out for me?"

"Why do you think, Sherlock? Because you're my friend, my best friend, and I care about you."

It didn't click until the early hours of the next morning.

Sherlock went into the sitting room, opened up the laptop and typed it in.

Password: Sherlock

Password correct. Logging in.

Sherlock smiled. Maybe this caring lark wasn't so bad after all.