Our 6 month old, Venus, caught sight of herself in the glass on the front of the woodstove and was fascinated. She would stare and wag her tail then look around the stove as if to see if the other puppy was hiding on the side. She did this for quite a while, alternating the sides she looked on. My husband reached over and waved his hand behind her and she started barking at this, it was really funny to watch. Her sister doesn't seem to notice the glass but Venus watches herself quite often but this was the longest it kept her attention.
Harley will find a mirror in the house and bark at herself. She jumps on the couch just to see the "other" dog. They do the cutest things. Unfortunatly every time I go to take a picture she looks at me instead of her reflection.