A guide to canine mouth signals:
2. Yawn: While it is usually interpreted by humans as meaning fatigue or boredom, it can actually be a stress-related signal, best interpreted as "I am tense or anxious."
3. Lips curled to expose some teeth, mouth still mostly closed: "You are annoying me!" This is the first sign of menace or threat.
4. Lips curled up to show major teeth, some wrinkling of the area above the nose, mouth partly open: "If you do something that I might interpret as a threat, I may bite." This is the next stage of threat but may also indicate fearfulness. Pressing a dog at this stage may lead to an aggressive attack.
5. Lips curled up to expose not only all of the teeth but also the gums above the front teeth, visible wrinkles above the nose: "Back off!" This is the full threat display that indicates a dog is ready to release a violent attack. If you are ever confronted with this display, you should not turn and run: the level of arousal is so high that your movement will probably produce a pursuit-and-attack response. Instead, cast your gaze slightly down (a slightly submissive eye position), open your mouth a bit (a bit of a counter-threat), and back off slowly.