Teaching a discipline requires making features of content, practices and epistemological stances visible to, and learnable by students. Coming to understand and generatively work with mathematical equations as models of natural phenomena in physics is not trivial for students. Moreover, it often requires extensive unpacking. In this talk we derive parallels between levels of reading comprehension and levels of understanding and generatively using mathematics equations in physics. Specifically, we argue that using mathematics equations in this manner involves deciphering the explicit and implicit meaning of the equation. For many students this does not happen intuitively and requires an epistemological change that needs to be necessitated. We illustrate our claims using ethnographic accounts of mentoring sessions in long term (12-18 months) research apprenticeships of high school students (11^th and 12^th grades).