You can breathe through your nose and/or mouth.
While you breathe through your nose, tongue should lie at the roof of mouth and stick there with gentle pressure. This pulls the base of the tongue ahead opening the airway and strengthens tongue preventing it from falling back and obstructing airway.
During nasal breathing, lips should be lightly in contact and head in natural position.
Correct tongue position along with nasal breathing is important for normal growth of jaws and face. Normal growth of jaws promotes straight teeth and a perfect smile.
Tongue Posture changes when you breathe through your mouth.