It's the first stage of the Psyco-Developmental Stages . Babies find gratification in sucking and swallowing, their focus is in the mouth because that's how they satisfy hunger (oral incorporation) and then they move to biting and chewing ... another stage

The term "psychological oral incorporation" refers to the psychology related to an infant's pleasure in nursing. Oral incorporation relates to the gratification that is generated by an infant from sucking and swallowing. reference： http://www.answerbag.com/q_view/2049105#ixzz0soAqZ7Kj