They'd found it while they were searching for some obscure piece of evidence. It was hidden behind a garbage skip, making pathetic sounds, and seemed thrilled to see them. Sherlock scooped it up and tucked it in his pocket, completely forgetting about what they were searching for.
He cooed at it the whole cab ride home.
When they arrived back at the flat, John filled up a saucer with milk, which they actually had for once. Two days before, Sherlock had done some sort of experiment that required five different types of milk, but he only used about a tablespoon from each. John didn't bother asking anymore.
The kitten quickly lapped it up with its sandpapery tongue.
"She," Sherlock told John. "It's a girl."
"We're not keeping her," John informed him, somewhat disappointed himself. But they'd babysat a kitten once, and Mrs Hudson quickly put an end to that. There was now a no pets rule at the flat. Not that Mrs Hudson's rules really mattered, but John wasn't looking for an increase in rent. Again.
"I know," Sherlock murmured, stroking her with his finger as she purred. "But I seem to be experiencing gigil."
John smiled a little.
"As am I," he replied, scooping the kitten up and cuddling it close to his chest. "Perhaps Mrs Hudson will feel the same way."