John and Sherlock had both recovered from their colds, but Sherlock had begged Mummy to let John stay over just one more night. They had an idea for a prank on Mycroft that Sherlock really wanted to try, but he couldn't tell Mummy that. Instead he said he wanted someone his own age to play with because Mycroft was boring. It was sort of true, close enough that John didn't give it away as a lie.
Now it was nearly midnight and the entire house was asleep except the two boys. They sat up in Sherlock's room, giggling quietly and waiting. Once the grandfather clock down the hall started to chime Sherlock gave a nod, signaling John to start moving. John reached under the bed and pulled out a pot of cold tea while Sherlock grabbed some rope and a bucket from his bookshelf. The pair scampered down the hall to Mycroft's room. Sherlock could hear his brother snoring through the slightly open door. Perfect.
John gave a devious grin. "I'll grab a stepstool," he whispered, handing Sherlock the pot of tea. While John went to find the stool, Sherlock poured the tea meticulously into the bucket, making sure not to spill any. He didn't have to worry about noise much, Mycroft slept like a rock. John reappeared by his side a second later with a chair dragging quietly behind him.
Sherlock tried not to giggle as he climbed up on top of the chair. "Pass it," he hissed impatiently. John made him wait while he fastened the rope first to the doorknob and then to the bucket's handle. Then he finally handed up the bucket. Sherlock balanced it carefully so when Mycroft got up the next morning it would pour all over him but the bucket itself would catch before hitting his brother in the head. John insisted on that, he'd accidentally his sister doing the same thing, and he'd been in trouble for ages.
Once the bucket was up, John tugged Sherlock down and pulled the stool away. Mycroft's breathing changed, he snorted in his sleep, and the boys gave each other a startled look before sprinting back to Sherlock's room.
They collapsed on Sherlock's bed in fits of silent laughter, imagining Mycroft's face the next day. After the laughter had died away, John's stomach made a loud grumbling sound. Sherlock grinned at it. "Here, I've got a surprise," he said proudly, making John cover his eyes. Then Sherlock reached over behind a pile of books and pulled out a plate.
"S'mores!" John whispered. Sherlock gave him a scolding look.
"You peeked!" Sherlock pouted. He'd wanted to surprise him. John only smiled in response, snatching a s'more off of the plate.
"Don't be grumpy, it's a great surprise!" John said through a mouthful of graham crackers and chocolate, marshmallow gumming up his words. Sherlock couldn't help but laugh at that, and then he took a bite of his s'more. John was hard to really surprise, but he was going to manage it someday.