To develop a model of students' debugging processes, I conducted a qualitative analysis of young students engaged in debugging computer programs they had written in the programming language Scratch. I present a microgenetic analysis that tracks how one student's attention to elements of computer program state shifted during his debugging process. I present evidence that this student had relevant domain knowledge and claim that his changing attention within the problem, and not his domain knowledge, mediated his debugging process. I hypothesize that a key competence in debugging is learning to identify what elements of program state are important to pay attention to and that this attention, and not only domain knowledge, mediates the debugging process. This hypothesis is consistent with a model of physics reasoning and learning from the Knowledge in Pieces theoretical framework and in this research I build upon education research outside of computer science. The case study analyzes the debugging process of a student entering the sixth grade, but I document an isomorphic case from a pair of college students to show that this pattern extends beyond this age.