A dog interprets the world predominantly by smell, whereas a human interprets it by sight. A human has about 5 million scent glands, compared to a dog, which can have anywhere from 125 million to 300 million (depending on the breed). In one example is: a small drop of blood was diluted into 20 Olympic sized swimming pools, the test dog could smell with ease that there was blood in the pool.
When dogs smell something they are not just registering a smell, they get an entire story. They can smell pheromones, which are not only found in urine and faecal matter, but on skin and fur. From this they can tell a lot about another dog or animal including if they are male or female, what they ate, where they have been, what they have touched, and even what mood they are in. They can also smell some forms of cancer in people, including smelling it “in situ” or at stage zero, mostly through human breath.