Sigmund Freud would say that an adult who smokes heavily or one who constantly bites her nails "is fixated at the oral stages because these needs were not met in infancy."
During the oral stage, the newborn child's essential wellspring of cooperation happens through the mouth, so the establishing and sucking reflex is particularly critical. The mouth is imperative for eating, and the baby gets delight from oral incitement through satisfying exercises, for example, tasting and sucking.