I thought that perhaps something came through the air-vent, down the bell-rope and on to the bed . Then there was the milk - and of course, snakes drink milk. It was easy for the Doctor to get Indian animals. And because he was a doctor, he knew that this snake's poison is difficult to find in a dead body. So every night he put the snake through the air-vent, and it went down the bell-rope on to the bed . Of course, nobody must see the snake, so every night he whistled to call it back. The sound of metal falling was the door of the metal box, which was the snake's home. Perhaps the snake came through the air-vent many times before it killed Julia. But in the end it killed her. And Helen, too nearly died because of this snake. But tonight, when I hit the snake on the rope, it was angry and went back through the air-vent. And so it killed the Doctor. I'm not sorry about that.