Mouth Breathing:

Breathing should be through the nose. However with allergies and some conditions it can seem impossible. This can cause (especially in children) facial distortion and dryness.

Open Bite:

All upper teeth should make contact with lower teeth when biting. However in an open-bite, the back teeth all make contact, but the front teeth do not.

Tongue Thrust:

In a normal swallow the tongue goes up to the roof of the mouth. However with tongue thrust, the tongue pushes forward

Thumb Sucking:

Can cause distortion of the pallate.