John found Sherlock in the chemistry classroom with Ms. Hooper, attempting to bargain with her for some of the potassium she had demonstrated in class. He rolled his eyes. Ever since Mycroft had locked their feral pet kitten in Sherlock's room, ripping apart his pillows and curtains, he'd been looking for a way to get back. Revenge, he'd called it.
"All I'm saying is that it's school property and the demonstration in class today was to warn you about the dangers of potassium when in contact with water when we use it in an experiment in class tomorrow," Ms. Hooper explained, but John could see that she was wavering under Sherlock's arguments. Despite the utter insanity of the request to borrow some potential explosives, Sherlock had always been good at debating and persuading. John had always wondered why he hadn't joined the debate team.
Sherlock had explained his expert plan to get back at Mycroft to him during a round of passing notes at the back of class. Apparently his fire alarm system was very sensitive, and if he placed the potassium around random spots in his brother's room and got the sprinkler system to come on… BOOM. Revenge accomplished.
Sherlock was obviously getting annoyed since the plan wasn't coming to fruition as he'd hoped, though only the slight wrinkle of his furrow brow was there to suggest his frustration. John leaned against the doorframe.
"Hey, it's lunch time and your mum is going to kill you if you don't eat, you matchstick," John finally said, startling both his chemistry teacher and his friend.
Sherlock glared at him for the interruption, but started moving toward the door regardless, knowing he had lost the battle. "As long as they're not serving that godforsaken grilled cheese again, I'll think about it."
"Grilled cheese is great, and you're insane."
"More like an unrecognized genius."