Many science educators encourage student experiences of "authentic" science by means of student participation in science-related workplaces. Little research has been done, however, to investigate how "teaching" naturally occurs in such settings, where scientists or technicians normally do not have pedagogical training and generally do not have time (or value) receiving such training. This study examines how laboratory members without a pedagogical background or experience in teaching engage high school students during their internship activities. Drawing on conversation analysis, we analyze the minute-by-minute transactions that occurred while high school students participated in a leading environmental science laboratory. We find that the participation trajectory was based on demonstration-practice-connect (D-P-C) phases that continually recurred in the process of "doing" science. Concerning the transactional structures, we identify two basic conversation patterns--Initiate-Clarify-Reply (I-C-R) and Initiate-Reply-Clarify-Reply (I-R-C-R)--that do not only differ from the well-known Initiate-Reply-Evaluate (I-R-E) patterns previously observed in science classrooms, but also could be combined to constitute more complex patterns. With respect to the organization of natural pedagogical conversations, we find that there were not only of "preferred" and "dispreferred modes" of responding but also "ambiguous dispreferred" modes; and the formulating organization not only includes "self-formulating" but also "other-formulating." These natural pedagogical conversations helped, on the one hand, students to clarify their understanding and, on the other hand, technicians (or teachers) to teach toward different needs for different students in different contexts. (Contains 3 tables and 6 figures.)