Most animals have a complete digestive system, with a mouth at one end and an anus at the other. Which end forms first in ontogeny is a criterion used to classify animals into protostome and deuterostome.
The human mouth is covered by an upper and lower lip. They play an important role in speech (it is part of the vocal apparatus), facial expression, kissing, drinking (especially with a straw), and smoking. Infants are born with a sucking reflex, by which they instinctively know to suck for nourishment using their lips and jaw. Lips are often adorned with lipstick or lip gloss although in most human cultures this is an affectation for females only. The philtrum is the vertical groove in the upper lip, formed where the nasomedial and maxillary processes meet during embryo development. When these processes fail to fuse fully, a hare lip and/or cleft palate can result. The nasolabial folds are the deep creases of tissue that extend from the nose to the sides of the mouth. One of the first signs of age on the human face is the increase in prominence of the nasolabial folds.
According to western etiquette, the mouth is kept closed, especially when chewing.
The mouth starts digestion by physically chewing the food and breaking it down with saliva.