Researchers have argued that other people provide not only great opportunities for facilitating children's learning but also great risks. Research on pedagogical reasoning has argued children come prepared to identify and capitalize on others' helpfulness to teach, and this pedagogical reasoning allows children to learn rapidly and robustly. In contrast, research on epistemic trust has focused on how the testimony of others is not constrained to be veridical, and therefore, children must be prepared to identify which informants to trust for information. Although these problems are clearly related, these two literatures have, thus far, existed relatively independently of each other. We present a formal analysis of learning from informants that unifies and fills gaps in each of these literatures. Our analysis explains why teaching--learning from a knowledgeable and helpful informant--supports more robust inferences. We show that our account predicts specific inferences supported in pedagogical situations better than a standard account of learning from teaching. Our analysis also suggests that epistemic trust should depend on inferences about others' knowledge and helpfulness. We show that our knowledge and helpfulness account explains children's behavior in epistemic trust tasks better than the standard knowledge-only account. We conclude by discussing implications for development and outline important questions raised by viewing learning from testimony as joint inference over others' knowledge and helpfulness.