An audible sound made in response to a stimulus .
Infants have a cry reflex that is a normal, involuntary response to a stimulus.
Older children and adults cry for emotional reasons, such as pain, fear, sadness , or embarrassment. Premature infants frequently do not have a cry reflex. The most common reason for a baby to cry is that he or she is hungry. Infants can cry for 1 to 2 hours each day for the first few weeks of life and at 6 weeks of age may cry for 2 to 4 hours per day. No one fully understands why infants cry so much, but many believe that crying helps infants expend energy and may be a reflection of the normal development of the central nervous system .