It is well-known that infants learn their native languages with incredible ease. I just came across a passage that puts this into particularly striking context:

A first point to note here is the obvious intellectual limitations that children have while language acquisition proceeds apparently without any effort. We are all extremely impressed if a two-year-old figures out to put the square blocks in the square holes and the round blocks in the round holes. Yet somehow by this age children are managing to cope with the extraordinarily difficult task of learning language.

This is particularly impressive, the authors point out, given that according to a number of theories

we are to believe that children do both of these things using the very same domain neutral intellectual resources. THis is all the more remarkable given that a complete grammar for a single language remains an uncompleted goal for professional linguists.

Laurence, S., Margolis, E. (2001). The poverty of the stimulus argument. British Journal of the Philosophy of Science, 52, 217-276.