When your best friend catches you in a bad mood, does she try to console you, give you space to cool off or lick her own face in an uncontrollable slobber? If your best friend is a dog, this third reaction may be familiar to you. Certain cuteness-obsessed Internet communities call it a "mlem"; some animal behavior researchers prefer to call it mouth-licking, and offer many possible explanations for the quirky canine behavior. Mouth-licking has been described as a stress-coping mechanism, a spontaneous display of arousal or a way to communicate desire to play with a certain toy or munch a certain treat. But according to a new study by animal behavior researchers from the University of Sao Paulo, Brazil, mouth-licking may actually be one of a dog's best tools for reading and responding to human faces -- in particular, angry faces.