Dr Roylott was a clever and dangerous man. He knew India well. So he knew that some Indian snakes had poison that could not be found in a dead person's body. He also knew that the poison was very quick, and this also helped him. The bite of the snake left only two little marks in the skin of the person that the snake bit, and the police did not see them. Then I thought of the whistle. Of course Dr Roylott had to call back the snake before the morning. He had trained it, by giving it milk, so that it returned to him when he whistled for it to come. He put the snake through the ventilator late at night. He was sure that it would come down the rope to the bed. Perhaps it would bite the person in the bed, perhaps it would not. She might escape every night for a week, but in the end she must be bitten.

"I had decided this before I entered his room. When I looked at his chair I saw that he had been standing on it a lot. Of course he had to do that to reach the ventilator. When I saw the safe, the saucer of milk, and the loop which he used to hold the snake, I was completely sure that I was right. The noise of something made of metal that Miss Stoner heard when her sister died was because her stepfather quickly closed the door of his safe to lock the snake inside. Once I had seen that my idea was right, you know what I did next. When I heard the snake I quickly lit the light and attacked it."