  We asked students to explain the structure and execution of their small
programs after they had submitted them to a programming exercise. These
questions about learner's code (QLCs) were delivered at three occasions in an
online and open course in introductory programming as a part of the digital
learning material. We make inductive content analysis to research the
open-ended text answers we collected. One third of the students struggled to
explain their own program code. This estimates possible occurrences of fragile
learning at the moment when a student seemingly succeeds in a program writing
exercise. Furthermore, we examine correlations between the correctness of the
answers with other learning data. Our results indicate that answering properly
aligned QLCs correctly has stronger correlation with student success and
retention than merely submitting a correct program. Additionally, we present
observations on learning event-driven programming to explore QLCs' potential in
identifying students' thinking process.
