Meaning: The dog is expressing its happiness towards a person it does not know by moving its tail quickly.
Choice of Words: Dog is an animal often kept as a pet. Wags means to move the tail back and forth quickly. Tail is the long, thin part of an animal's body that extends from its back. Happily means feeling or showing pleasure or contentment. At is used to show the direction or target of an action. Stranger is a person who is not known or familiar.