Apprenticeship and the associated support mechanism of scaffolding have received considerable interest by educational researchers as ways of inducting students into science. Most studies treat scaffolding as a one-way process, where the expert supports the development of the novice. However, if social processes generally and conversations specifically are dialogical in nature then we would expect to observe two-way processes. The purpose of this paper is to report the results of an ethnographic study of high school students' internships in a scientific laboratory. Data were collected through observation, fieldnotes, and videotaping. Drawing on discursive psychology and conversation analysis, we find that laboratory technicians and students draw on different forms of discursive strategies to articulate knowledgeability while transacting with each other. We put forth the notion of "emergent expertise" to describe new forms of expertise that are not a property of individuals but rather the product of collective transactions. Our study illustrates the importance of opportunities generated in the internship for both old-timers and newcomers to bring about knowledgeability. This study implies a rethinking of the role of the expert and the notion of scaffolding, which puts more emphasis on the transactional process rather than on learners as recipients. (Contains 1 figure and 1 table.)