So this might surprise you, but one of the most amazing feats you'll ever accomplish as a human being already happened, and that is language development. I mean, think about it. When you're a baby, all these sounds are coming at you, and somehow, you're able to figure out which sounds are words, where there are breaks between the words, general grammatical rules, and you're able to apply them without any real formal training. This is amazing. So naturally, a lot of research has been done into how this ability develops. And I'm going to tell you about the three main theories that look at language development. So first, we start out with the nativist, or innatist perspective. And what this perspective says is that children are born with the ability to learn language. And the main guy associated with this theory is Noam Chomsky. And he thought the humans had something called a language acquisition device, or LAD, in their brains that allowed them to learn language. And this isn't really supposed to be in a specific part of the brain. It's just an idea that this ability exists. And this works because he thought that all languages shared a universal grammar, or the same basic elements, so all languages would have nouns, verbs, things like that. So the language acquisition device enables the child to pick up on and understand those types of words and their organization within a sentence for any language. This goes along with the idea that there is a "critical period" or a "sensitive period." The "critical period" is usually thought to be from birth until about age eight or nine, and it's the period of time in which a child is most able to learn a language. So if you try to learn a language after that age, it's a lot harder. It's not impossible. It's just a lot harder. And nativists like Chomsky would say that that's because the LAD only operates during that critical period. Once you start using it, then it specializes to your language, and it becomes unable to detect other sounds and grammar from other languages. The second theory I want to tell you about is the learning theory. Learning theorists think that children aren't born with anything. They only acquire language through reinforcement. So a learning theorist would say that a child learns to say "mama" because every time it makes it sound that approaches that-- so "ma-something"-- then Mom starts smiling, hugging the child, so over time, the child learns, oh, the more I make this sound, the more I get hugs and smiles. And so then, eventually, it learns to say "ma," and then say it again, and learns to say "mama." So this makes sense. But a strict learning theory doesn't explain how children are able to produce words they've never heard before or produce unique sentences. So we have another theory called the interactionist approach. Sometimes this is called the social interactionist approach, because these theorists believe that biological and social factors have to interact in order for children to learn language. So they would say that children strongly desire to communicate with others, such as the adults in their lives, and that desire motivates them to learn to communicate via language. And the main theorist associated with this school of thought is Vygotsky. He was a big proponent of the importance of social interaction in the development of children. All three of these theories have made big contributions to our understanding of how children develop language. So the next time you look at a baby, be impressed. They're actually working really hard.