How we change what others think, feel, believe and do
Teeth body language
There's not a lot of body language with teeth, but this is a complete section, so here's details.
Teeth are made to bite, tear and grind. Exposing the teeth in a snarl is saying 'I am thinking of biting you' and is hence a primitive and potentially scary threat.
Actual biting is rare, but indicates the person has been reduced to a base position and is probably not thinking rationally.
Biting can also be affectionate, for example where lovers chew the other person's lip or ear. This can create arousal from the basic fear instinct but the person knows from the context that they are not in danger and hence reframes the arousal as pleasure. Gentle biting also stimulates nerve receptors and is thus similar to touch.
Biting the inside of the cheek or biting the lip can indicate suppression of a desire to speak. Lip biting or sucking can also be a comforting action when a person is anxious (perhaps as a substitute for thumb-sucking).
Young animals at play pretend to bite one another as they prepare for adulthood. When people play with friends they may also expose their teeth.
Exposing the teeth in smiling tends to indicate extreme pleasure. People who are self-conscious and particularly if their teeth are not that attractive may try not to show their teeth when smiling.
Teeth can make a noise when banged or slid together. Chattering teeth may indicate extreme fear and is usually accompanied by shaking of the body. This may also indicate extreme coldness.
Grinding teeth can indicate suppressed anger or frustration as the person tensely tries not to speak.
Light tapping of the teeth can be mild frustration or thinking (it is similar in effect to tapping of a finger).
As with other repetitive action, teeth noise can also just be habit.
Sometimes people tap their teeth with their nails, making a noise that echoes in the mouth. This can signal thinking or boredom. It may also be a deliberate interruption or irritant, although this is less likely.