In Euclid’s Elements, the exterior angle theorem (Proposition 1.16) states that the measure of an exterior angle of a triangle is greater than either of its remote interior angles. This theorem is valid in Euclidean geometry and hyperbolic geometry, but not valid in spherical geometry. Indeed, the proof for this theorem originally presented in Elements contains a hidden assumption of absolute geometry and can be modified in modern systems (e.g., SMSG axioms). In a college geometry course, students constructed Euclid’s proof for the exterior angle theorem in a spherical model using dynamic geometry software. They searched for counter-examples of the theorem on the sphere to explain when and why this theorem does not hold on spheres, namely what makes this proof fail. In this talk, I will present how this proof analysis task engaged students in exploring figures and generalizing their observations.