My twin girls are fast approaching age two. Language is coming at them from everywhere, and sticking like crazy.
There are people who believe that you can't really think without the facility of language. I'm pretty sure I'm one of those people. I heard a guy on the NPR science-y show, Radio Lab, theorize that language acquisition actually creates neural pathways in the learner that would otherwise not be blazed. I don't know about all that--I guess it seems feasible--but regardless, I can't see how you could really ponder over any matter without words, or some kinds of symbols, to identify all the players, relationships, actions, and contexts in whatever drama you were contemplating. It seems like you could know things from having experienced them; but you couldn't really work out any problems in your head. Memory would also be tricky without language.
So as the kids are learning to talk, their thoughts are becoming more organized. They're applying the knowledge gained from past experiences to their current situations. They're using some primative logic and argumentation.
Also, they're revealing what's on their minds. And let me tell you, almost-two-year-olds think about some deep shit.