Around their first birthday, many babies say one or two simple words such as hi, dog, Dada or Mama. But babies begin to understand words before they say them. They learn words like cup and shoe, and they respond to simple requests. Learning language is more than making sounds. Listening, understanding and knowing the names of people and things are all part of it.

The work begins with sound. Babies can hear loud sounds from the start, and they may turn their head toward familiar voices. In 1980, researchers showed that newborns would change how they sucked to switch on a recording of their mother reading a nursery rhyme. Newborns begin to recognize important sounds around them, such as the voice of a caregiver. Babies learn the sounds of the language that surrounds them, and by 6 months most recognize its basic sounds. As babies grow, they also begin to sort out the speech sounds that make up the words of their language.

Babies also make sounds of their own, and they do it in stages. In the first few months, up to about 4 months, babies coo, making soft pleasure sounds like oooo and aahh. Babbling is when babies make strings of sounds like tata, upup and bibibi, and it starts in the first year of life. Around 6 months many babies take turns making sounds with a grown-up.

Babies begin to smile back at people who talk and smile to them, usually within the first few months. Around 4 months many babies smile on their own to get a grown-up's attention and chuckle. Around 6 months they laugh.

Babies cry to tell the people near them what they need, such as food, a hug, a clean diaper or sleep. A familiar, gentle voice can calm a crying baby.

Turn-taking has a name: serve and return. It is a back-and-forth between a baby and a caring grown-up, like a game of tennis. A baby coos or babbles, the adult answers, and the baby learns. When a baby babbles, gestures or cries and an adult answers with eye contact, words or a hug, the baby's brain builds and strengthens connections. The human brain expects this back-and-forth, and it is essential for healthy development.

Naming what a young child is seeing, doing or feeling helps make language connections, even before the child can talk or understand words. The first 3 years of life are the most intensive period for acquiring speech and language skills, and there appear to be critical periods when the brain is best able to absorb language.

How do researchers find out what a baby knows about words? In a famous 1996 study, infants heard about 2 minutes of an artificial language and were then able to pick out the words in the stream of speech. Researchers showed that babies could tell the artificial words from nonwords by measuring how long the babies listened in a head-turn preference procedure. Studies like this suggest that infants use statistical information in the speech stream to find where words begin and end.

A second kind of experiment studies expectations instead of sounds. In violation-of-expectation studies, infants see two test events. One matches what they expect, and the other breaks the expectation. Before the test events, infants may see habituation events that get them used to the test situation. With the right controls, reliably longer looking at the unexpected event than the expected event is evidence that infants hold the expectation, notice the violation and are surprised by it.

Why is the brain so ready for all of this? In the first few years of life it grows new connections at a very fast rate. It contains billions of connections between neurons, which allow lightning-fast communication among neurons that specialize in different jobs. Genes provide the blueprint for the brain's circuits, but repeated use strengthens the circuits used most often, based on a child's experiences. The brain is built from the bottom up, with simple connections and skills developing first, followed by more complex circuits and skills. As of November 2026, Singapore's Health Promotion Board says a baby's brain roughly doubles in weight in the first year.

The brain's ability to adapt is called plasticity. It is greater in a child's earliest years and declines as we age. After a period of especially rapid growth in the first few years, the brain refines itself through pruning, which makes its circuits more efficient. The early years are the most active time for making neural connections, but new connections can form throughout life.

Words to know

• plasticity: brain's flexibility
• babbling: strings of sounds
• pruning: trimming extras
• habituation: getting used
• neurons: brain cells