Holmes stood at his chemistry set, doing nothing in particular except passing the time and trying to stay quiet. Watson was asleep on the settee behind him, finally resting for the first time in days, and Holmes wanted him to continue sleeping. He deserved the rest after staying up with Holmes both on a case and through the case's unfortunate aftereffects.
He picked up a new agent he had found a few days before and began experimenting with it, making notes as to which chemicals it reacted with and how after checking the chemical equations, a frequent pastime of his. Why rely on others' observations when he could use his own?
He must have made a mistake on one equation, however, because it did not produce the reaction he had been expecting.
Immediately after introducing the reactant, the mixture started bubbling when he had expected it to change color. Before he could neutralize the reaction, it exploded in a burst of light and sound, sending a cloud of fumes into the air.
Coughing, he threw open a window, expecting the half-asleep doctor to grumpily join him as they waited for the air to clear. Watson would be grumpy both over being woken and for falling asleep to begin with. The doctor had intended to run to the pharmacy an hour ago, but Holmes saw no reason to wake him.
It was only when the air began to clear that he realized he was alone at the window, and he spun around.
Watson lay on the settee, apparently still asleep, and Holmes frowned. In the few years they had roomed together, Watson had woken at a moments' notice for patients and the occasional midnight case. How could he have slept through that explosion when the smallest sound from Holmes stuck in a nightmare would have him coming down the stairs?
"Watson?"
"Hmm?" was the sleepy reply.
"Alright, Watson?"
Watson hummed an affirmative, still mostly asleep, before cracking an eye open. "Why?"
Watson registered the fumes at the same time that he focused on Holmes' disheveled appearance, and he coughed a few times, burying his head in the back of the settee.
"What'd you do, Holmes?"
Holmes barely understood the slightly muffled question and shrugged, his focus still on his flatmate. "Reaction played out differently than expected. Do you sleep through explosions often?"
Watson huffed a still half-asleep laugh. "Only at war and near consulting detectives."
Watson still had his face turned toward the settee, and Holmes let the smile escape. "And you say I sleep like a rock."
"No," came the correction. "I said you sleep like a cat."
Holmes thought about that. What did a pesky feline have to do with his sleeping habits?
Watson cracked an eye open again at the silence. "Up to twenty hours a day," he clarified, "and you ignore anything that wakes you up."
Holmes' quiet bark of laughter went unnoticed. Watson was already asleep again.