Although tutoring by expert human tutors is usually effective, it is not always. By contrasting cases where tutoring does and does not result in learning, we can find out what causes learning during tutoring. Approximately 125 hours of physics tutorial dialog were analyzed to see what features of the dialog were associated with learning. Successful learning appears to require that the student make an error or reach an impasse; too much help can prevent learning. Features of successful tutorial explanations appear to be different for different pieces of knowledge. For instance, some pieces of knowledge are learned only if the tutor emphasizes generalization, whereas other learning requires that the tutor first explain why the student's error is wrong.