The vocabulary of a language — the words themselves — is the product of a project that spans the history of a culture and can involve millions of person years of work. Every sense of every word records some intellectual discovery that now outlives the myriad other, less distinguished thoughts that never earned a name.
Each person invents some new ideas, but most of these will die when their owners do, except for those that make their way into the culture's lexicon. Still, from that ever-growing reservoir we each inherit many thousands of powerful ideas that all our predecessors found. Yet it is no paradox to say that even as we inherit those ideas from our culture, we each must reinvent them for ourselves. We cannot learn meanings only by memorizing definitions: we must also understand them. Each situation in which a word is used must suggest some mixture of materials already in the mind of a listener, who then, alone, must attempt to assemble these ingredients into something that will work in consonance with other things already learned. Definitions sometimes help — but still one must separate the essences from the accidents of the context, link together structures and functions, and build connections to the other things one knows.
A word can only serve to indicate that someone else may have a valuable idea — that is, some useful structure to be built inside the mind. Each new word only plants a seed: to make it grow, a listener's mind must find a way to build inside itself some structure that appears to work like the one in the mind from which it was learned.
Along with the words, we also have to learn the grammar-tactics for using them. Most children start by using only one or two words at a time. Then, over the next two or three years, they learn to speak in sentences. It usually takes a full decade to learn most of the conventions of adult speech, but we often see relatively sudden advances over concentrated periods of time. How do children learn such complicated skills so quickly? Some language theorists have suggested that children learn to use grammar so readily that our brains must be born with built-in grammar-machinery. However, we've seen that our visual-systems solve many similar problems in even earlier years — and we've also seen that when they learn to play with spoons and pails, children must learn yet other languagelike skills for managing the Origins, Destinations, Recipients, and Instruments of their actions. Thus, many sections of our brains appear to demonstrate capacities for rearranging pronome roles even before we learn to speak. If so, perhaps we ought not to wonder so much about how children learn to speak so readily. Instead, we ought to wonder why it takes so long, when they already do so many similar things inside their heads.