Combining a few prompts this time:

9. From trustingHim17 - Include one of Holmes' chemistry experiments
10. From trustingHim17 - Describe a prank war or other amusing day at Baker Street
11. From Hades Lord of the Dead - Inspector Lestrade has a personal problem
13. From W. Y. Traveller - "I cannot forgive this."
14. From W. Y. Traveller - Breaking point.

"Urghk!" Holmes leaned backward, choking on the thick, acrid smoke arising from his chemistry set. He had been performing a simple test to see if the green stains found on the victim's coat were from grass or his sister's dyes - vital knowledge to narrow down the suspects. There was nothing in there that should have caused such vile smoke.

Waving a hand in front of his face to keep the smoke at bay, he examined the beaker closely. There were curious grey specks floating in the solution, ones he recognised, but had certainly not introduced himself. There was no call for gunpowder for this test.

His eyes narrowed. Only one person would have devised a prank like this.

"Philip!"

Three days earlier

Lestrade stood in the living room of 221B Baker Street, wringing his hands. "I didn't want to bother you gentlemen, but I have no one else to turn to."

"A new case?" Watson asked, with a side glance at Holmes. It had been several days without anything to attract Holmes' attention, and Holmes' boredom had gone from endless scraping at his violin to an obsessive attention and deduction of Watson's every move. A case would be much welcomed.

"Not a case, Watson, or our dear inspector would not be so hesitant," Holmes drawled from his position lounging across the sofa. "Surely a personal matter. Not a medical one, or at least not one requiring your assistance, Watson."

"No. It's about my son." Lestrade hesitated, casting an anxious glance back at the doorframe. "I shouldn't impose on you. Please forget I was here." He started for the door, only to be called back by Holmes.

"My dear Lestrade, whatever your request, at least allow us the courtesy of hearing it before you assume our disinterest. We have no case at present, and I confess to curiosity as to what your son could have done to require our particular expertise." Holmes gestured to a seat, and Lestrade sunk into it, visibly resigning himself to seeing his request through.

"It's not anything he's done, Mr Holmes. Philip's a good boy, if a bit high-spirited. But my wife's visiting her sister on the Continent, and now I'm being called up to Edinburgh to consult on a case, just for a day or two, but I'm afraid he'll find himself in some mischief." Lestrade drew a hand across his brow. "He could stay with my aunt, but her son - my cousin - is a bit of a bad lot, and is likely to draw Philip into some trouble. Which is why- it's a lot to ask -"

"You're asking if we would be able to keep an eye on Philip while you're away," Watson said, rescuing Lestrade from his stuttering. "Of course we can."

Lestrade blew a deep sigh of relief. "Thank you. I appreciate it more than I can say." With a short discussion of details, Lestrade left, his shoulders less burdened.

Philip's arrival the next day had been met with interest and some trepidation on both sides, which quickly changed to amusement. As Lestrade had promised, his son was polite and well-behaved, with a curious nature that provided Holmes sufficient distraction from his boredom as he introduced the boy to the wonders of chemistry.

It was on the second day, when Watson woke to find everything in their living room had been shifted two inches to the left, did he begin to suggest perhaps letting Holmes spend extended time with a small child had been a bad idea. Even then, conspiring with Philip to put salt in Holmes' coffee made it worthwhile.

Only when Gregson came by with a new case did the potential problems of the situation make themselves felt. Reluctantly, Mrs Hudson agreed to watch the boy for a few hours, but even when Holmes and Watson returned, the were unable to pay the same attention to Philip as they had previously, a situation which did not change throughout the rest of the day or the next morning. Judging by the smoke Holmes was still waving away, Philip had not taken it well.

"Watson!" He called sharply. He heard the creaking of the stairs, followed by the familiar sound of Watson's footsteps entering the room. "Watson, you know that we have been through many trials together, but this boy may be my breaking point."

Watson didn't answer, instead turning to the window and opening it, breathing deeply of the clearer air. "What happened, Holmes? I've never seen you make a mistake with your chemistry before."

"This was no mistake," Holmes said darkly. "This was a child meddling in what he does not understand, and because of it, a killer may escape justice. I cannot forgive this, Watson."

Watson, at first hiding a smile at Holmes' dramatics, quickly sobered as he realised what was at stake. "Are you able to repeat the test?"

"Perhaps. The remaining sample is very small - it may not be enough for a result."

"If anyone is able to do it, it will be you." Watson turned from the room. "I will find Philip, and explain what his actions may have caused."

Holmes nodded absently, already thinking the best way to use the minuscule remaining sample. His mind on the problem, he lost himself in his work, mixing chemicals carefully to ensure no trace of gunpowder remained. Finally, he held the solution up to the light, and watched as it turned a deep purple.

Leaning out the window, he whistled to the Irregular in the street below, who came racing up the stairs. Holmes quickly wrote a few lines on a piece of paper and handed it to the girl. "Give this to Inspector Gregson." The Irregular nodded, and raced back down the stairs even faster than she'd come up.

Finally, Holmes leaned back in his chair, a smile of satisfaction growing across his lips. A killer had been caught, disaster had been averted, and by now, a mischievous child would be the guilty recipient of one of Watson's rare but very effective lectures. All in all, the past few days had been a very effective relief for his boredom.