In this paper, we study the antecedents of moments of particularly successful learning while students use a Cognitive Tutor for geometry. Students used the Cognitive Tutor as part of their regular classroom activities and data was collected automatically. Learning moments were operationalized as when the probability that the student just learned was extremely high, as determined by a probabilistic model: the moment-by-moment learning model. The results indicate that while self-explanation is weakly predictive of learning moments, contextual guessing and several other factors are even better predictors of learning moments. These results suggest that unexpected events in student behavior may be good predictors of changes in knowledge.