This article uses a functional view of language to frame and analyze issues of language and content in mainstream classrooms. Describing a Western Canadian grade one/two science class, it examines how a teacher and her class of young ESL students were able to build up a simple theory of magnetism in a scientific register, link its technical terms to their practical experience, and apply the theory to explain and extend their experience of magnets. She thus created a new blend of theory and practice in their activity of doing science. The study demonstrates the value of a functional perspective on social practice, leading to a sharper understanding of issues of language and content learning in mainstream classrooms and a greater ability to analyze relevant data. Educational implications include a richer understanding of the connections between students' practical experience and their theoretical understanding.