"What are they like? The...Dark Dog Days?"
John had asked the question of him once, his voice halting as he repeated the words no doubt overheard from a rare telephone conversation.
Sherlock had only laughed bitterly, sending the other man scowling and returning to his newspaper.
It wasn't as if anything would have come of it had he answered - he had learned long ago that trying to explain the Dog to anyone who had never been in Its jaws was futile.
After all, how could John Watson be expected to understand the idea that if one were to cut a Holmes open on a Dark Dog Day, all that one could be expected to find was bile and black ichor and venom? It was medically impossible, of course, but for all intents and purposes, it might as well be true. How could Greg Lestrade understand the scars that criss-crossed his elder brother's skin when he saw them? How could Mrs. Hudson, or any other landlady who had the misfortune of leasing to him, be expected to explain the reasons behind periodic vanishing acts?
No, Sherlock would never explain the Dog to anyone. Mycroft knew, and that was enough.