"-and while it would be ever so fun to vomit up chunks of my esophagus, I would appreciate-" There was a pause, and Sherlock glanced away from his microscope for a moment when he heard a familiar, put-upon sigh. "You're not listening to me, are you?"

John had his back turned to the table, prepping the kettle with the economy of long practice. A glance at the slope of his shoulders and the slight cant of his neck was enough for Sherlock to determine that John was not truly irritated, and so he turned his attention back to the slide. He added another few lines to his sketch of tuberculosis bacteria, and then said, "I tuned out when the whinging began."

"You left a bottle of hydrochloric acid in the fridge. Unlabeled. It's not whinging to object to your attempts to poison me."

Sherlock's mouth drew into a moue of displeasure. Without taking his eyes away from the microscope, he exchanged a light red pencil for one of a deeper shade. With a few deft strokes, a plasmid appeared on the sketch, floating in the bacteria's capsule of cytoplasm.

"Really, John, I'm insulted. If I had it in my head to poison you, you would be poisoned. No attempting about it." His flatmate huffed and dragged down a pair of well-worn mugs, just as the kettle clicked. "Besides," Sherlock said, "it's only a 4 percent solution. Drinking from the bottle wouldn't kill you."

"Be bloody uncomfortable, though."

John couldn't possibly have seen Sherlock's shrug of indifference, but he seemed to sense it anyway.

"Fine," he growled, giving up, as Sherlock had known he would. "Just… label everything from now on, okay? We have guests sometimes, and I have no desire to help you hide a body."

"Not to worry. I know of nineteen ways to dispose of a corpse without detection." Sherlock looked away from the microscope and scowled. "Twenty-two if we involve Mycroft."

Something in Sherlock warmed when John laughed, and his frown had all but disappeared when his flatmate turned, a steaming cup of tea in each hand. "Impressive," John said, his voice full of wry warmth. "I can only think of about eight myself."

"That's why you will never be a successful criminal. Not enough contingency plans."

John huffed in amusement and handed over Sherlock's usual mug. The overfull cup sloshed as the detective gripped the handle, and a drop of hot liquid spilled onto the back of John's hand. With a muttered curse at the slight sting, John put his mouth against the offending droplet and sucked it away. He met eyes with Sherlock, his crow's feet deep slashes that betrayed his mirth, with the blade of his flattened hand drawing attention to the deep blue of his irises, like an underscore.

Inexplicably, Sherlock's grip on his mug loosened and a few fat droplets of tea fell onto his notebook, marring the careful sketch. He tore his gaze away from John, a more difficult task that he had anticipated, and dabbed carefully at the drops with the cuff of his dressing gown. His breathing was elevated, his heartbeat a sudden staccato in his ears, and he wondered absently if this was what a bout of hypertension felt like.

A couple of deep breaths chased away the feeling, and he looked up again to see John with a expression that was equal parts curiosity and worry. "All right?" he asked.

There were flecks of gold around John's pupils. How had he not noticed that before?

Feeling off-balance, Sherlock glared at his tea, as if it had personally betrayed him. "Of course."