This study aims to describe a 6th-grade student’s progress in understanding written proof texts through participating in an individual teaching experiment. The same task was administered to the student twice in an interview setting, before and a year after the teaching experiment. The student was asked to evaluate four arguments aimed to prove a given conjecture. While in the pre-interview, the student accepted all four arguments based on her Naïve Experience; in the post-interview, she rejected empirical arguments and looked for General Procedures (or Abstract Structures) that necessarily apply to the whole set of numbers under investigation. In the post-interview, spontaneous changes occurred in the student’s understanding of the given arguments after she constructed her own proof for the same conjecture. Instructional design elements used in the teaching experiment might have facilitated her understanding of the structure of deductive proof texts.