Why do dogs wag their tails?

Science is always providing new information that allows us to interpret the behaviors of dogs, or to reinterpret behaviors which we thought we understood very well-such as the meaning of a dog's tail wagging.
Perhaps the most common misinterpretation of dogs is the myth that a dog wagging its tail is happy and friendly. While some wags are indeed associated with happiness, others can mean fear, insecurity, a social challenge or even a warning that if you approach, you are apt to be bitten.