Welcome to my little 221B to celebrate Bonfire Night (or Guy Fawkes night - whichever you prefer) - a singularly British celebration of a failed plot to blow up King James 1 and the Houses of Parliament :)
Thank you once more to MapleleafCameo for checking this over.
Disclaimer: I own nothing...

"So tell me again Sherlock, how you are going to prove that the son deliberately killed his step-father?"

John scratched his head and stared at the collection canisters, metal and plastic, several balls of string in varying thicknesses, and a selection of electrical circuit boards and wire loops.

Sherlock sighed, and started to explain as if to a child.

"The step-father had refused to fund the boy's ambition to be a speedway racer- he wouldn't have succeeded anyway- and so he planned to make a noise loud enough to interfere with the man's pacemaker and cause it to fail."

"Right, I get it…" he paused, and then shook his head. "No I don't. What do you need all of this for?"

John really, it's obvious. I plan to replicate the makeshift bomb, and measure the effect on the electrical impulses of these circuit boards."

"They represent the pacemaker?"

The look John received said far more than any of Sherlock's scathing comments could.

Fortunately he was saved from further humiliation as his flatmate suddenly he raised his head.

"Aha! That sounds like Mrs Hudson with the last ingredient."

The diminutive landlady walked into the kitchen and with a cheerful smile placed three packs of sausages on the table. John frowned. Sherlock stared.

"Mrs Hudson." He wailed. "I asked for two dozen Bangers!"

A/N: Bangers are fireworks - small tubes of gunpowder with a touch paper, that serve no purpose other than to go bang! 'Bangers' is also a slang name for sausages - hence that great British favourite, Bangers and Mash. Poor Mrs Hudson...she gets so confused :(