The term "cry baby" is a bit of a misnomer, at least for the first weeks of life. Although newborns are physiologically able to produce tears, they almost always cry without them. No one knows exactly why this is, though some suggest immature neural wiring or that newborns simply don't have the emotional intensity necessary to elicit tears. (And, of course, they haven't seen Bambi yet.)
Babies are born with basal tearing, meaning their tear ducts deliver just enough wet stuff to keep the eyes moist and healthy, and reflex tearing, the kind you get when slicing an onion or battling with a stray eyelash. Newborns do not have psychic tearing (tearing that indicates emotional distress). It's between 2 and 4 months of age that a baby's psychic tears kick in. A trigger—loneliness, frustration—causes the baby's nervous system to stimulate a cranial nerve in the brain, and that in turn sends a message to the tear glands.
Be warned: A baby's dry-eyed wail is one thing, but add a little water to that puckered-up face and there won't be a dry eye in the house.
Adapted with permission from Why Babies Do That: Baffling Baby Behavior Explained, by Jennifer Margulis, published by Willow Creek Press. 2005 by Jennifer Margulis. All rights reserved.