Not-knowing is an underexplored concept defined as an individual's ability to be aware of what they do not know to plan and effectively face complex situations. This paper focuses on analyzing students' articulation of not-knowing while completing geometric reasoning tasks. Results of this study revealed that not-knowing is a more cognitively demanding task than knowing. It became evident that the students have difficulty expressing not-knowing. Through reflections, interviews, transcription analysis, and meaning coding four recurring themes emerged that could possibly determine why students have difficulty expressing not-knowing which will be further discussed in the paper.