2 . What is an object ?

trol and bad manners; they are unreflective artists and inept dance partners. However, they are strikingly good at learning the meanings of words. Children learn their first words by 12 months of age, are relatively proficient at word learning by 16–18 months, and eventually come to learn new words at a rate of over ten new words per day (see Box 1). Their early vocabularies include personal pronouns (me, you), proper names (Fido, Mommy), prepositions (in, on), adjectives (good, big), verbs (bite, want) and many classes of nouns including those referring to whole objects (dog, cup), substances (milk, water), parts (eye, finger), habitual activities (bath, nap), periods of time (minute, day) and abstract notions (story, game)1,2. While children sometimes get the precise meaning of a word wrong – for instance, sometimes calling a cat ‘a dog’ – serious mistakes are rare: children never call a chair ‘a dog’ or confuse proper names with common nouns, object names with substance names or adjectives with verbs3. One perspective on word learning is that parents do much of the work, carefully tailoring their speech to make the connection between words and what they describe particularly clear to their children. Such tutelage does occasionally occur in many cultures, including the middle-class Western culture that is the focus of most language acquisition research. But it is not universal; there are societies in which parents make no effort to teach words to children, leaving them to learn words on the basis of overheard speech4. Nevertheless, such children have no problem in developing a rich vocabulary. Furthermore, children raised in Western cultures learn at least some words, such as the personal pronouns5, by overhearing them in the conversations of others, and even the most pampered child will learn many words that are used when the relevant object or event