(Extracted from The Adventure of Silver Blaze on Wikipedia)
Gregory: “Is there any other point to which you would wish to draw my attention?”
Holmes: “To the curious incident of the dog in the night-time.”
Gregory: “The dog did nothing in the night-time.”
Holmes: “That was the curious incident.”
The “curious incident of the dog in the night-time” is easily explained: the dog made no noise, because no stranger was there. As Holmes explains: “I had grasped the significance of the silence of the dog, for one true inference invariably suggests others…. Obviously the midnight visitor was someone whom the dog knew well.”