Some days, Sherlock wished he hadn't been so insistent on his independence from his brother. Otherwise, he wouldn't have had to take on a flatmate.
The flatmate would have been alright, he supposed, if it wasn't for the complications. John was quiet, kind, and really had needed Sherlock's help. Sherlock was sworn to help those in need, despite how much he really didn't want to sometimes.
No, the problem was that Sherlock, who had lived in a household of open wizardry since before his found him, was going to have to hide his wizardry from his new flatmate, and that was going to cause a world of trouble.
Luckily for Sherlock, John wasn't terribly observant. Or if he was, he didn't question much, as long as it could be explained away. Sherlock went out of his way to make himself look as odd as possible, so that very little couldn't be explained by "well, he's Sherlock." This approach had worked well for him in the past, so it wasn't as hard as it could have been.
Sherlock's manual had recently been upgraded too. The slightly outdated-feeling and beaten up book was stored lovingly in an otherspace pocket, just in case. His new MacBook was fantastic, it was intelligent enough to keep up with him, and all it needed was the occasional trip a few universes over for updates and patches. They'd named him after one of his 'older sibs,' a member of a sentient computer species named Logo.
Logo and Sherlock got on well together, Sherlock was thrilled to find something sentient that could keep up with him. From Logo's descriptions of his 'family,' Sherlock privately thought he might have some trouble keeping up with them. Not that he would have told anyone.
John, on the other hand, may not have been able to keep up with Sherlock mentally, but he gave it his best shot. Sherlock was wizard enough to admit to how intelligent John was when compared to other people, but it was hard to live in a world where no one understood you. John gave it his best try, which is why Sherlock always felt extremely guilty whenever he lied to John about something wizardly related, making him grumpier as a result.
One day, soon after the case that John would infamously dub "A Study in Pink," Sherlock was sprawled on his couch with Logo, doing some research into the intricacies of the Speech. Sherlock's specialty was spell building. His mind was constructed such that he had a tendency to see answers to questions that others can't, and a way with the Speech that he was excellent at arguing down the Universe. This skill served him well in his other specialty: solving impossible problems. However, that skillset required almost constant, detailed research. This, thankfully, tended to keep Sherlock from getting too bored.
On this particular day, Sherlock was practicing some of the more unusual tenses with Logo. They were vaguely complex, as they were tenses dealing with some of the more obscure aspects of time that most humans can only vaguely understand. As usual for Sherlock, he basically ignored other people's comings and goings. Unfortunately for him, this was the day he wished he hadn't.
He heard a banging noise coming from the vicinity of the door.
"Shut up, I'm working," he said loudly and absentmindedly, not realizing he was still using the Speech. There was blessed silence for a few moments. Suddenly John spoke up.
"That wasn't nice Sherlock," he said reprovingly in the Speech.
"I'm working," he repeated absentmindedly. Then John's language use penetrated the single mindedness of his research. He looked up slowly to see John standing in front of him with a half smile on his face and his brown notebook in hand. Sherlock's eyes zeroed in on the notebook, then looked back at John.
"Dai stiho," John said wryly.
"Oh, God," Sherlock put his head in his hands, "How long have you known?"
"You and that computer aren't quiet at all," John said with a laugh, "I've known ever since it showed up." Sherlock groaned. He wasn't used to missing important details. Then he brightened.
"Well now you can help me out with wizardry problems too," Sherlock said, "If you want to that is."
"My specialty is healing right now," John commented wryly, "Knowing you, you'll need it."
"Dai stiho, cousin," Sherlock said with a grin.
"Dai."