"Mama" is easy to understand. Newborns had increased brain activity when they heard words with repeating syllables, according to a new study from the International School for Advanced Studies in Trieste, Italy. "We didn't see this increase in brain activity with words that didn't repeat," says study author Judit Gervain, Ph.D. Humans have likely had this ability throughout time, so as we evolved, we gave ourselves names our kiddos could say.

Frequency is less important. "If babies first spoke what they heard most, then they'd be saying 'the,' but they don't," says Twila Tardif, Ph.D., a professor of psychology at the University of Michigan in Ann Arbor. Relevancy matters. "Bottle" makes the Top 10 list, probably because it's something babies and moms care about, says Tardif. And words like "hi" and "bye" do, too, because parents often speak them in a very animated way, which gets a baby's attention.

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