Babies understand more speech than parents think and can link words to objects at a very young age, a study found.
At just six months they recognise the meanings of some words are more similar than others, researchers found.
For example young children were able to tell words like car and pram were more alike than words such as car and juice, the Daily Mail reports.
The researchers claim that parents should talk to their children as much as possible because they are always listening and learning from what you say.
As part of the study the researchers, from Duke University in Durham, North Carolina, showed babies images of linked objects while a caregiver was asked to name one of the objects.
Eye-tracking software revealed the young children then spent more time looking at the image that was named.
Professor Elike Burgelson, lead author of the study, said: "Even though there aren't many overt signals of language knowledge in babies, language is definitely developing furiously under the surface.
"Even in the very early stages of comprehension, babies seem to know something about how words relate to each other.
"And already by six months, measurable aspects of their home environment predict how much of this early level of knowledge they have."
Professor Burgelson also discovered babies' word knowledge was linked to the amount of time they listened to people talking about objects in their immediate surroundings.
For the first part of the study she invited babies and their carers into a lab fitted with a computer screen and few other infant distractions.
The infants were shown pairs related images, like a foot and a hand, or unrelated images, like a foot and a carton of milk.
For each pair, the caregiver, who couldn't see the screen, was told to name one of the images while an eye-tracker followed the baby's gaze.
The youngsters spent more time looking at the named images when the two images were unrelated than when they were related.
Professor Burgelson said: "They may not know the full-fledged adult meaning of a word, but they seem to recognise that there is something more similar about the meaning of these words than those words."
She then studied the babies at home and discovered babies learned to recognise words linked to objects in their surroundings.
Her team looked at the different aspects of speech the babies were exposed to, including the objects named, what kinds of phrases they occurred in, who said them, and whether or not objects named were present and attended to.
She said: "It turned out that the proportion of the time that parents talked about something when it was actually there to be seen and learned from correlated with the babies' overall comprehension."
Study coauthor Professor Sandra Waxman said: "This study is an exciting first step in identifying how early infants learn words, how their initial lexicon is organised, and how it is shaped or influenced by the language that they hear in the world that surrounds them."
Professor Waxman added more research is now needed to understand how parents should speak to their babies.
She added: "Before anyone says 'this is what parents need to be doing', we need further studies to tease apart how culture, context and the age of the infant can affect their learning.
Prof Burgelson said: "My take-home to parents always is, the more you can talk to your kid, the better.
"Because they are listening and learning from what you say, even if it doesn't appear to be so."