Those supercute coos are airy sounds that come straight from the larynx--making them easy to say for tiny babies still figuring out how to use their lips and tongues. They're also fun. "Kids tend to focus on particular sounds: squeals, vowels, or growls, as we call them," says D. Kimbrough Oller, Ph.D., a professor of audiology and speech-language pathology at the University of Memphis. These will help your little one learn to control vocal tone and volume--something she'll need to form her first word.
What to say back: Anything in "parentese," that singsongy voice that sounds like it came off a children's CD--only it's you repeating "Hiiiii!" Research shows the high pitch makes your infant really take notice of--and want to imitate--what you say.