Sherlock Holmes often explains his reasoning to Watson at the end of a tale, after the mystery has been solved and the guilty party is either dead or has been taken off to jail. Another good example is found in "The League of Red-Headed Man." In "The Adventure of the Speckled Band," Holmes explains his reasoning after Dr. Roylott has been killed by his own snake. Part of Holmes' explanation includes the most important clues and the deductions derived from them. Holmes admits that he was somewhat on the wrong track until he was able to examine the room in which Helen Stoner was sleeping and the adjoining room occupied by her stepfather Dr. Roylott.
My attention was speedily drawn, as I have already remarked to you, to this ventilator, and to the bell-rope which hung down to the bed. The discovery that this was a dummy, and that the bed was clamped to the floor, instantly gave rise to the suspicion that the rope was there as a bridge for something passing through the hole and coming to the bed. The idea of a snake instantly occurred to me, and when I coupled it with my knowledge that the doctor was furnished with a supply of creatures from India, I felt that I was probably on the right track.
The fact that the bed was securely fastened to the floor was perhaps the most important clue of all. From it Holmes deduced that Dr. Roylott wanted to make sure that the snake would end up on the sleeping girl's bed when it crawled through the ventilator and down the dummy bell-rope. Holmes deduced that it could only be a snake that could do this. The facts that Roylott was a doctor and that he was interested in exotic animals from India suggested to Holmes that the doctor kept a snake whose venom was undetectable in an autopsy. The useless bell-rope and useless ventilator were also highly suggestive of some ulterior purpose.