How do babies learn to talk?

Babies can learn whatever language (or languages) they're exposed to. But how do they do it? It's a step-by-step process of learning that begins even while they're still in the womb.

A baby's ears are completely formed by the 24th week of pregnancy, but can start hearing some noises around Week 18. "This isn't particular words," says Sudha Arunachalam, director of Boston University's Child Language Lab, "but they can hear sort of the up and down, the intonation of the language."

Arunachalam points to a study by French and German scientists that shows that newborn babies cry differently, depending on what language they've been exposed to in utero.

"French newborns," Arunachalam said, "cried with rising intonation patterns. German babies cried with a falling intonation pattern. That goes with the musical qualities of French and German more generally."

Hear the cries at 18:54 in the audio below:

After being born, babies continue to absorb the language around them. In their first couple of months, they start to make vowels. At 4 to 6 months, they begin what is known as "vocal play."

The first consonants they add to their repertoire are generally G, K, M, P, D.

"Sounds that tend to be easiest are M, P, D — and the hardest are R and L," said Arunachalam. "The P and M sounds you make with your lips closed and with the R and L you have to put your tongue in kind of funny places. Babies also practice by making those raspberry sounds."

At 7 to 8 months, babies start to put more vowels and consonants together.

"This starts to sound like the language they're learning," Arunachalam said. "They're putting them together in things that sound like words and sentences."

Then, at 10 to 12 months, a baby might say his or her first word. The first word is often a name for a person or object — something like mom, dad, dog, book, or ball.

And even if they can't say a lot of words, babies understand a lot more than they can say.

"Through eye-tracking studies, we do know they understand the meanings of some everyday words like milk, nose, banana," even before they can say them, said Arunachalam.

Before babies can talk, they also can use some basic sign language — either simple signs taught to them by their parents or more basic ones, like pointing.

By about 18 months, babies are putting two words together. These aren't complete sentences, but the idea of a sentence is there — just missing some little pieces.

These little pieces, like is or the, are added to a baby's speech in a specific order, laid out by researcher Roger Brown. This is known as "Brown's 14 Grammatical morphemes."

First comes -ing ("Mommy drivING"), then in and on, followed by regular plurals (cups or cats). The last one to be added is the contractible auxiliary, which is the ability to turn "Mommy is driving the car" into "Mommy's driving the car." See the full order here.

These small parts of speech are added during the time a child is around 2 to 4 years old.

Then, by the time children are 4 or 5, they are able to talk like little adults.

So if there's a baby nearby, remember that baby is learning from you.

"They understand so much more than they say," Arunachalam said. "And they are listening very, very carefully. They are listening for patterns in what you say and they're picking up on so much. So it's important to remember that they're listening and they might understand more than you think they do."

For more from Brains On, the MPR News science podcast for kids and curious adults, you can subscribe in iTunes or wherever you listen to podcasts.