It's ridiculous how people refer to it as chemistry. Of course it is chemistry, but that is not what they mean when they try to sum up the minutia of micro-expressions that cross the faces, postures, and hand movements of two people staring at each other in a room. They don't mean his chemistry, the chemistry of tattered lab notebooks and chemical separations that filter out a clue within what was thrown out as one's man's rubbish. Sherlock hit the "mute" button on the TV, killing the sound of an interviewed actor using that word yet again to describe his relationship with someone else paid to pretend on camera. Ridiculous.

Love, as they call it, can be reduced to a simple chemical equation: C8H11NO2+C10H12N2O+C43H66N12O12S2. It even looks appealing, somewhat symmetrical in a visual way. Sherlock sometimes considered drawing it in pencil on the plainer of 221 B's wallpapers. No one would mind a little note-taking on the walls after the bullet holes and high-pigment spray paint. He wrote it out in the margins of John's abandoned newspaper crossword puzzle, trying to avoid the impulse to fill in the blanks left by the doctor during his rushed breakfast. Like the night sky, he could appreciate the formula even if it did nothing for him.

Sherlock's experiments in sentiment reached back to age fourteen when, after receiving a sharp blow to the side of the head by a girl somewhat shorter than him, he was informed of his utter lack of human feeling. "You have no heart at all," she had spat at him, grabbing her viola case as she headed to the door. She hadn't appreciated the knowledge that her older boyfriend was cheating on her with her best friend.

He began methodically by finding items which made him happy or calm for reasons unrelated to their immediate use or value. His martial arts diploma reminded him of the tight discipline of his private training, the delicious empty mind that came after practising the same poses over and over to trim perfection. The diploma was no longer necessary; surely a simple demonstration held enough proof of Sherlock's skill. Still, he kept it above his bed as something to look up at in the night when swirling thoughts left him wanting for steel-trap concentration.

It took him three years of online research, but he found the discontinued laundry washing powder used by the housemaids before he went off to school. It faded his nicer shirts too quickly, but he used it on his pyjamas because of the aching nostalgia behind the smell. John had held the box up with his usual questioning look aimed at the 70s-style packaging, but the word "experiment" proved enough to assuage his curiosity.

"Just don't set anything on fire this time," he had cautioned, setting the powder down on the counter.

Though he detested them, even the slighted taste of Walker's crisps was enough to remind him of the thrill of his first case. The younger Detective Inspector Lestrade had thrown the prawn-flavored monstrosities at his head when they found the killer in the exact flat Sherlock predicted. The flush of success had been enough to blow the keen rush of drugs out of the question, and the cheap snacks still brought a memory of that elation.

What Sherlock didn't expect was to create more specific associations in his chemical examinations.