We contribute to the understanding of teacher noticing by focusing on what a teacher may notice in students' mathematical contributions in the context of problem-based lessons. Complementing approaches to research on noticing that focus on individual teachers' perceptual, cognitive, or situated skills, this conceptual article offers four categories of perception as examples of affordances available in the practice of teaching mathematics through problems. These include (1) the familiar instructional situations available to frame the problem, and the possibility to see student's work as (2) responsive to the problem, (3) serviceable for the knowledge at stake, and (4) normative with respect to the instructional situation used to frame the problem. The article shows examples of how teachers recognize responsiveness, serviceability, and normativity of student contributions and calls for research that can further uncover how such recognition may matter in the practice of teaching.