This article is a version of the talk given by Jonathan Osborne as the Association for Science Education (ASE) invited lecturer at the National Science Teachers' Association Annual Convention in San Francisco, USA, in April 2011. The article provides an explanatory justification for teaching about the practices of science in school science that form a core element of the "Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas" published in July 2011 by the National Research Council of the US National Academies. (Contains 2 figures and 2 online resources.)