Puzzle pieces are so cliche, and Sherlock doesn't want to think of himself and John like that. Besides, whoever heard of a puzzle with two pieces? (And puzzle pieces come apart, can be attempted to fit together with another, John had done that before, he wasn't patient enough for puzzles and tried to pound them in place. Sherlock felt like other people had done that to him before. Perhaps not that bad?...)
No, it was more like... Sherlock didn't even know. Yin and yang? Left and right ventricles? Ventricles and atriums? Thymine and Adenine? Guanine and Cytosine?
It was more than that though. It was like they were parts of each other, couldn't be split.
DNA? No, DNA splits and replicates, not at all accurate.
Fire and ice, water and air, land and sea. There can't be one without the other. There is no light without the dark. No stars without the nighttime sky. (He can still appreciate it.)
It's simply a unit. They are a unit. Sherlock and John. Holmes and Watson.
Simply.
