Dogs have an olfactory system that is far more complex and sensitive than that of humans. They have up to 300 million olfactory receptors in their noses, compared to our mere 6 million, and their brains are able to process smells 40 times faster than ours can. When they take a series of short sniffs, they are effectively taking a "sniffing snapshot" of the air around them and sorting through all the different smells they are taking in. This allows them to identify a particular scent more quickly and accurately.