The mouth is an orifice in the head. It contains teeth, the tongue and the palate. A mucus membrane lines the interior, which transitions into the lips. The back of the mouth features the tonsils and uvula. The mouth serves multiple functions. Food enters the body via this entrance, where the teeth assist in the chewing process. Saliva begins the digestion process, before the food travels down the esophagus and into the stomach. The orifice can also work as an air passage every time the body inhales and delivers oxygen to the lungs. In breathing, only the nose presents an alternative route. Also, the mouth is critically important to human communication. The voice box produces sound, but mouth shapes sounds into consonants and vowels. This occurs in how the tongue is positioned and whether or not it touches the teeth or the palate. The mouth also plays an active role in non-verbal communication. The mouth conveys emotional states through by moving the lips into different positions.