People have often asked me whether I knew the end of a Holmes story before I started it. Of course I did. One could not possibly steer a course if one did not know one's destination. The first thing is to get your idea. We will suppose that this idea is that a woman, as in the last story, is suspected of biting a wound in her child, when she was really sucking that wound for fear of poison injected by someone else. Having got that key idea, one's next task is to conceal it and lay emphasis upon everything which can make for a different explanation. Holmes, however, can see all the fallacies of the alternatives, and arrives more or less dramatically at the true solution by steps which he can describe and justify.
He shows his powers by what the South Americans now call 'Sherlocholmitos', which means clever little deductions, which often have nothing to do with the matter in hand, but impress the reader with a general sense of power. The same effect is gained by his offhand allusion to other cases. Heaven knows how many titles I have thrown about in a casual way, and how many readers have begged me to satisfy their curiosity as to aigoletto and His Abominable Wife', 'The Adventure of the Tired Captain', or 'The Curious Experience of the Patterson Family in the Island of Uffa'. Once or twice, as in 'The Adventure of the Second Stain', which in my judgment is one of the neatest of the stories, I did actually use the title years before I wrote a story to correspond.
Sometimes I have got upon dangerous ground, where I have taken risks through my own want of knowledge of the correct atmosphere. I have, for example, never been a racing man, and yet I ventured to write 'Silver Blaze', where the mystery depends upon the laws of training and racing. The story is all right, and Holmes may have been at the top of his form, but my ignorance cries aloud to Heaven. I read an excellent and very damaging criticism of the story in some sporting paper, written clearly by a man who did know, in which he explained the exact penalties which would have come upon all concerned if they had acted as I described, Half would have been in jail and the other half warned off the turf forever. However, I have never been nervous about details, and one must be masterful sometimes. When an alarmed editor wrote to me once: 'There is no second line of rails at this point,' I answered: 'I make one.' On the other hand, there are cases where accuracy is essential.
( Very amusing story about the snake in "The Speckled Band" )