ET, the well-known Extra-Terrestrial, learnt human language fast: "His ear-flap opened and he listened intently. ... His ... circuits buzzed, assimilating, synthesising. ... Thus inspired, the language centre of his marvellous brain came fully on. ..." Yet ET's magical ability is almost matched by that of human children. As the American statesman Benjamin Franklin once said: "Teach your child to hold his tongue; he'll learn fast enough to speak."

Children talk so readily because they instinctively know in advance what languages are like.Language has a biologically organised schedule. Children everywhere follow a similar pattern. In their first few weeks, babies mostly cry. From six weeks onwards, infants coo or even mew. From around six months, babies babble language-like sounds. Single words - "oo!", "da!" - are produced from around the age of a year.

Children don't at first realise that sounds can be labels for things. Early words are tied strongly to a location, and often relate to a whole scene. A word "da" for a toy duck might be for one particular duck as it floats in a particular bath. Only later will "da" be used for a duck away from the bath, and later still extended to all ducks, and maybe swans, geese - and even toy boats. The naming insight, the discovery that things have names, is a major leap forward. Children pass this milestone at various times, typically before the age of 18 months. Parents don't usually notice it, it seems so normal, because adults expect things to have names.

The naming insight is followed by a "naming explosion". This eruption in vocabulary leads to word combinations, "mummy push", "daddy car" and so on. Some phrases are novel, as "bye-bye sock", "all gone kitty", which are unlikely to have been copied from adults. By the age of three, children utter long sentences, though some things, such as pronouns, still cause problems. Three-year-old Adam said his doll "shuts she's eyes", instead of "shuts her eyes".

At around three and a half, children talk freely. By this time, they have acquired most of the constructions used by adults. A few gaps still exist for all children up to the age of around 10.

Just as bees learn fast to distinguish flowers from, say, balloons or bus stops, so human children are preset by nature to pick out natural language sounds: they don't get distracted by barking dogs or quacking ducks. Their learning is innately guided. Inbuilt signposts direct youngsters, so they instinctively pay attention to certain linguistic features, such as stressed vowels and word order.

A biological time-clock ordains the sequence in which the language web is woven, though not the exact dates. Language acquisition begins well before the age of two. Babies only a few days old can pick out their own language, according to some French research, as shown by increased sucking movements when familiar sounds are played to them. Infants still in the womb may become accustomed to the rhythms of the language spoken around them.

And language development does not come to a shuddering halt at adolescence: vocabulary even undergoes a spurt at this time. So the idea of a fixed critical period is now disputed. Yet most people find it easier to learn languages when they are young - so a sensitive period may exist, a time early in life when acquiring language is easiest, and which tails off.

A "natural sieve" hypothesis is one idea put forward to explain this. Very young children may extract only certain limited features from what they hear, and may automatically filter out many complexities. Later learners may have lost this inbuilt filter, and be less able to cope as everything pounds in on them simultaneously. A "tuning-in" hypothesis is another possibility. At each age, a child is naturally attuned to some particular aspect of language. Infants may be tuned in to the sounds, older children to the syntax, and from around 10 onwards, the vocabulary becomes a major concern. Selective attention of this type fits in well with what we know about biologically programmed behaviour.

Language holds together in a network of implications. If a language has one type of construction, others are predictable from it: if, as in English, a language has verbs before its objects, as in "climb the tree", then it will also probably have prepositions before nouns as in "up the tree". A language such as Hindi or Turkish would have the reverse, and say, as it were, "the tree climb", and "the tree up".

But natural web-spinning can be both helped and sometimes hindered by the speech of those around. Parents pick on etiquette, "Say please", or swear words, "Don't let me hear you say that word again", or occasional pronunciation problems: "Say Trisha, not Twisha". If they do pick on language formation, it's often verb endings: this may be useful, if the child is tuned in at that time to learning these. If not, the correction is likely to be ignored.

But even with face-to-face contact, the young learner sets the agenda. Clear, varied utterances directly addressed to the youngster are the strands out of which the child builds the language web. The talk has to grab the child's attention. Joint enterprises are all important. Parents often find it easier to talk to girls, mainly because they involve them more often in domestic chores: "Come and help mummy with the potatoes", mothers often say to their daughters.

So children build the language web by extracting what they need from the talk they hear around them. Most are efficient chatterers long before they go to school. But they still need to learn which type of speech to use when - so-called "communicative competence": babies and bank managers must be addressed in different ways, just as different clothes are required for the beach and a wedding.

The language web, then, has been mostly acquired by children by around the age of 13. At this age, language suddenly becomes a mudslinging match between generations, at least in England. Teenagers want to talk like their pals, but parents disapprove. Matiness and casualness are sometimes emphasised by swearing. F-words swarm like bees in some recent literature, and buzz about freely in conversation. Today's swear words are undergoing a bleaching process, a fading of meaning that happens in all semantic change. In the last century, oaths using the name of God were widely disapproved of. Then they gradually lost their power to shock. These days, f-words and s-words no longer horrify so many people.

As with spiders, a lot of time and effort has to go into the weaving process. But humans, unlike spiders, can think about the webs they've woven. This sometimes gives rise to a superfluous cobweb of worries. Ideally, the final layers of a child's web-building would be supplemented by two extra, conscious strands: tolerance of minor variations and an interest in each other's speech.

The writer is the Rupert Murdoch Professor of Language and Communication at Oxford University. Her Reith Lectures, 'The Language Web', are broadcast on BBC Radio 4 on Tuesdays at 8.30pm.