Disclaimer: nothing mine. Conan Doyle and BBC share them.
Unexpectedly
It comes as a surprise. Well, two surprises. First there's Donovan becoming a single parent. Now, don't go and start hating on Philip. It's not his. They might have a off-again on-again friends-with-sometimes-benefits relationship, but he has at least brain enough to be careful. In truth, Sally won't say who the other parent is, and you don't try to pry it out of her if you want to be safe. She's perfectly capable and willing to rip you apart. The father is out of the picture, and that's it. Not even Sherlock has dared to voice that it was obviously a drunk one night stand, because...Or whatever else his deductions on the matter might be. That one, honestly, is Greg's theory.
Then – when little Mark is born – the bigger, is-doomsday-approaching surprise happens. Sherlock volunteers to babysit, in days when he has no cases, if Sally needs it. There's no love lost between them, so why would he offer to help?
"You're not hurting my child," she hisses viciously.
"Of course not," Sherlock replies, affronted. "I can manage what is in the capacity of any fifteen years old girl. Leave me instructions if you wish, useless as it will be. I'm not going to hurt him unwittingly, and purposefully is way below me."
"You don't turn him into a freak," she orders then.
"I'm not contagious. John should be evidence enough," he counters flatly.
"Fine." Being a single parent is hard. She needs all the help she can get. Even the Freak's, since he's so keen on that.
When they're home, John simply has to ask why. What is Sherlock's aim?
"Anything is better than being bored," he replies.
"Only that? Really?" the doctor inquires. Because...babysitting?!
"Oh. Fine. It's an experiment," Sherlock huffs.
"Sherlock." John's voice has his warning, this-is-very-much-not-good tone.
"It's not harmful, John," the detective whines.
"Explain." It's an order. Captain Watson doesn't need to say it.
"Brain development can be stimulated right from the start. It's perfectly useless to spend months cooing at the child."
"You want to make Mark a genius." John's voice has shifted to his 'you're amazing' inflection.
"We have to account for his genetic makeup too. Genius might be asking too much from Donovan's son." The venom when mentioning Sally is back. He had to refrain himself from giving an adequately disparaging comeback to her words earlier after all, and her insults where entirely undeserved. "Not an idiot would be enough."
"I'd be happy to help with that experiment," John announces, smiling blindingly at him.
"Obviously. I already counted on you."