This study used responses of 20 student teachers to describe the different ways in which student teachers perceive the practice of their mentors as internal triggers for learning in their practicum experiences in the schools. Reported observations from pedagogical journals of student teachers were described according to various kinds of teaching knowledge and skills and were further categorized as providing either support or challenge to student teachers' perceptions of learning to teach. The student teachers identified support and challenge in each one of the categories of teaching knowledge and skills, thereby supplying evidence for different kinds of emotional and cognitive triggers for learning. Based on the findings, we make some recommendations for supervision of mentoring relationships. (Contains 3 tables.)