"I've got it!" Sherlock pounded his fists on the table, eyes twinkling with giddy glee. John, who had fallen asleep watching telly, started.
"What?" He was having trouble wrapping his head around…well, anything. It'd been a long day at work—crying children, people insisting he was wrong with his diagnosis, being coughed in the face as he was checking someone's tonsils, lab equipment breaking down, and the like.
Sherlock whipped his head around to John, quite happy, as John rarely saw him. His hands, uncharacteristically, were marked with chemicals and cuts. "Come over here," he demanded. (Sherlock Holmes rarely asked anything nicely.) John stood up and made his way to the kitchen table, which was covered in various cleaners, blood-soaked straight pins, and the usual microscope slides and mystery bottles. Sherlock grabbed two pieces of wood and stabbed himself with the pin.
"Sherlock, what—" John should have known better than to be surprised at anything Sherlock did any more, but he really couldn't help it.
"Watch," was all he said, as he dripped blood onto a corner of each piece of dark wood. He then produced a bottle with a dropper, and dripped a yellow liquid in a different corner. After that, bleach, and after that, horseradish, with the final addition to the board being something John had no idea what it was. Sherlock waited for each of the substances to dry, and then gently sprayed each block of wood with one of two chemicals. (He's doing this in our kitchen?) He then turned out the lights and waited for their eyes to adjust.
One block, all six substances glowed dimly. The other, only the blood did. "What?" asked John in the darkness. Sherlock switched the lights back on, grinning like a kid who's getting free all-you-can-eat ice cream.
"Onto each of these blocks, I've put blood, urine—"
John was startled. "Whose urine?"
"My urine, obviously." Sherlock was irritated at the interruption.
"Well, in that case, you need to drink more water." John said this as a medical professional rather than a friend. Sherlock just stood quietly for a few seconds before continuing.
"Blood, urine, bleach, horseradish, and a copper-based wood treatment. Luminol will react with all of these, as you just saw, which makes it difficult to determine whether or not a substance was actually blood. It's caused numerous mixups and been the defining factor in quite a few cases in which the defendant was let off because of reasonable doubt. But this," Sherlock said, holding up one of the mystery bottles, "It will revolutionize forensic science! It only reacted with blood. It's perfect."
"Wait, hang on, you invented that?" John hadn't realized that Sherlock's chemical skills were that great.
"Don't sound so surprised." Sherlock looked a bit hurt. John was meant to share in his giddy glee, not react with confusion.
"I'm not—that's great, I just—"
"Didn't expect me to be brilliant at chemistry."
"No—no, not that—I don't even know anymore," he finished lamely and with a sigh. Sherlock, too, sighed, disappointed in his protégé for not realizing the importance of the discovery.
"John, I've taken one of the staples of modern forensic technology and improved upon it. I'd have thought that would warrant some sort of enthusiastic response." The excitement had left his face, giving way to the child of irritation and disappointment.
"I'd be more likely to throw a party if I had a full night's sleep," John remarked.
"Fine then, go to bed. Maybe Lestrade will have the right response."
Great, thought John. He's sulking again. Just what I need. And, shaking his head, John went upstairs to his actual bed, with Sherlock excitedly describing the results of his experiment to Lestrade.