John's trying to mind his own business. Sherlock is an adult, he can take care of himself. But honestly, those noises are getting increasingly more alarming, and there's that godawful smell to worry about.
Rubbing his hands over his face, John grumbles and runs down the stairs where he's engulfed in a cloud of yellowish smoke, and the horrifying odour has increased tenfold.
"Sherlock? SHERLOCK! Bloody hell. What is this?" Blindly, John fumbles into the kitchen, where he finds Sherlock looking sheepish and holding the top of a shattered beaker.
"Sulphur..." As if this is remotely enough of an answer to explain the horror in the kitchen.
It's at this moment that Mrs. Hudson decides to yell up the stairs. "Boys? What is that ruckus?"
John groans and leans out the doorway. "Uh, nothing, Mrs. Hudson. Just one of Sherlock's experiments."
"Well it smells dreadful. Just be sure to open a window, would you?" She tuts and shakes her head, ducking back into her flat. John lets out a relieved breath he didn't realise he'd been holding. He was sure that this time would be the last straw, that she'd finally run out of patience and kick them out. It wasn't the first time, nor would it be the last time, that John would be infinitely grateful for their landlady's benevolence.