Children typically acquire a few words very slowly and then around 2 years of age undergo a 'word explosion.' A cognitive scientist at the University of Iowa has built a mathematical model which suggests that this amazing phenomenon is not genetically controlled, as has long been thought. Instead children experience a burst in learning because language consists mostly of words that are medium-hard to learn. There are many fewer easy and hard words. The word spurt is just a byproduct of the learning process and the nature of language. The pattern of slow growth up until a crucial threshold is, apparently, true for many domains in which children learn, such as music, art, and athletics. Why Files, New Scientist, Bob McMurray.