Leaving nothing but slobber marks behind, that other baby sure is fun. Your baby wonders, "She was silly. She touched her nose when I did...she laughed when I did."
To find this out, researchers use a little test in which they dab a colored spot on the baby's face and then let her loose in front of the mirror. If she touches the spot or tries to rub it off, she must know that the reflection she's looking at is her own physical body. If she continues to babble, spurt and play as normal, she hasn't fully grasped the concept. Most babies don't pass this until they're at about 18 months.
It's not until around age two that babies develop self-awareness, realizing that they're a separate being, physically and mentally. So, up until then, looking in the mirror is a social activity, meaning to her, it's really like having a friend that lives in the shiny surface.
Mirrors definitely help in their development though, so encourage those babbles of communication between baby and reflection (even if it does mean having to constantly clean the mirrors).
Does your baby like to look at herself/himself in the mirror?