This study sought to examine why peer interaction can facilitate learning, with the hypothesis that collaborative learning provides a social context that is conducive to the generating of explanations (an activity positively associated with learning). Individualistic and collaborative learning contexts were compared for 96 college students (19 male, 77 female) instructed either to "talk aloud" or to "explain" as they learned about the human circulatory system from a text (a 2x2 design). Although the subjects in the two learning contexts did not differ in learning scores, generating explanations (defined as new inferences that went beyond the text material) was highly predictive of both factual and conceptual learning. Motivation and other types of talk were also examined. Motivation was related to factual learning but not conceptual learning. An attachment contains the instructions to subjects in the learning study. (Contains 2 figures and 12 references.) (Author/SLD)