I own nothing!
This is based off of an idea I had in the lunchroom. Probably not actually mine originally.
John sighed as he walked up to the door of 221 Baker Street. He was absolutely exhausted. The clinic had been packed, with several unnecessarily loud women, and he had had another row with the checkout machine at Tescos when he had bought groceries on the way home. Bloody Sherlock's fault for getting kicked out of all the Tescos… he grumbled to himself as he fumbled with the keys. Finally, finally he got in through the door. He kicked it shut behind him. Mrs. Hudson was out to a sister's birthday party, and wouldn't be back for another day, so no help there. He sighed as he walked up the stairs to the flat he shared with the world's only consulting detective.
"John, do not step into the flat." Sherlock was reclining on the couch, hands steepled under his chin. John stopped short at the doorway. "Why… Sherlock, what nasty chemical ended up on the floor this time?" he sighed wearily. Sherlock turned to him, eyes wide. "The floor, John! The floor is lava!" John blinked. "…What?" "The floor of the interior of our flat is molten lava, John. Surely you agree that is an unsafe surface to walk upon?" John blinked again.
Then he slowly smiled. This was quite possibly the last thing he had expected, but Sherlock sure as hell needed a break from the latest case (John wasn't sure when his flatmate had last slept, but it wasn't in the past week). So, he carefully put the bags near the door, took off his shoes, and leapt onto the couch with a THUD. Sherlock looked at him approvingly. "Good. You might just survive." "So, are we safe here, or are some of the other furnitures lava too?" Sherlock sat up and looked around. "The lava spawns in the kitchen. As such, it has been entirely submerged, and the lava is flowing into this room… We ought to get onto the armchair, John." So they traveled, careful not to touch the floor as they crawled onto the armchair. John giggled.
Gregory Lestrade sighed as he stepped up to the door of the flat. He knew Sherlock was hiding something from them on this last case, and had therefore organized a 'drugs bust' in hopes of getting him to… explain whatever he had miraculously found. Anderson, Sally, and some others had eagerly volunteered. He walked in through the door, thankful that Mrs. Hudson was away. She would surely fuss over the officers, and he did not need a distraught Mrs. Hudson. He traipsed up the stairs, as noisy as he could be in a (probably futile) effort to intimidate the detective. He noticed the bags of groceries and shoes near the door… odd. He opened the door.
"Alright, drugs bust" Lestrade said loudly as he opened the door and walked in. "NO!" two voices chorused. Lestrade blinked. There on top of the armchair was John, and Sherlock was perched precariously on the top of the couch. They were both grinning. "What the-" "Lestrade! Get off the floor! The floor is LAVA!" cried Sherlock. "What?" "Too late, he's dead" sighed John, turning to Sherlock. "Nobody else step into the flat!" John was using his most authoritative, army captain voice and no one moved. "Guys, I don't know what you're doing, but this is a drugs bust." replied Lestrade in his most authoritative voice, which wasn't too shoddy either. Sherlock's eyes widened. "But you can't come in! You, Lestrade, are already dead and the lava will kill anyone else who goes into our flat!" "Alright Freak, what's going on?" Sally had elbowed her way past an incredulous Lestrade. "You're dead," said Sherlock matter-of-factly, "because the floor is lava and you're just standing there. It spawns in the kitchen, by the way, so I wouldn't try to go into there." Sally stood there, openmouthed. "WHAT?!" Sherlock giggled.
"Lava, John!"