Abstract: Tinkering has been shown to have a positive influence on students in open-ended making activities. Open-ended programming assignments in block-based programming resemble making activities in that both of them encourage students to tinker with tools to create their own solutions to achieve a goal. However, previous studies of tinkering in programming discussed tinkering as a broad, ambiguous term, and investigated only self-reported data. To our knowledge, no research has studied student tinkering behaviors while solving problems in block-based programming environments. In this position paper, we propose a definition for tinkering in block-based programming environments as a kind of behavior that students exhibit when testing, exploring, and struggling during problem-solving. We introduce three general categories of tinkering behaviors (test-based, prototype-based, and construction-based tinkering) derived from student data, and use case studies to demonstrate how students exhibited these behaviors in problem-solving. We created the definitions using a mixed-methods research design combining a literature review with data-driven insights from submissions of two open-ended programming assignments in iSnap, a block-based programming environment. We discuss the implication of each type of tinkering behavior for learning. Our study and results are the first in this domain to define tinkering based on student behaviors in a block-based programming environment.