Reading a review of Sousa's How the ELL Brain Learns led me to this excellent paper. On the face of it, the paper addresses a very narrow issue: that a distinction may be drawn between basic interpersonal communicative skills (BICS) and cognitive academic language proficiency (CALP). But it covers much wider ground, exploring the ways in which language-learning is not a single skill, but a set of interrelated skills. If, for example, you compare a six-year old and a 12-year old, "there are enormous differences in these children's ability to read and write English and in the depth and breadth of their vocabulary knowledge, but minimal differences in their phonology or basic fluency." I think applies more generally. 'Knowing physics' on the baseball field or the billiards table is very different from 'knowing physics' in the laboratory, even if exactly the same underlying principles are at play.