I will review two very different studies that probe undergraduates' conceptual understanding, knowledge organization, and transfer of knowledge in mechanics. The first study employs a new approach termed "problem posing," whereby high-performing students pose mechanics problems from "problem situations" or "concept scenarios." From the problems posed, coupled with students' explanations, we can deduce how novices organize and cross-reference physics knowledge in memory, and the extent to which they are able to apply knowledge flexibly across different contexts (i.e., transfer knowledge). In the second study, students make judgments about which of several simulations best depicts what would actually happen when steel balls are released on differently shaped tracks. Finding suggest that newly learned physics knowledge interferes with students' ability to make appropriate judgments.