This study was conducted on informal aspects of an inquiry-based physics course and reports findings about learning interactions and discourse observed during the first three semesters the course was offered. The course offered an alternative to the large lecture instruction typical in introductory university physics and promoted learning in an informal environment. The course organization attempted to engage students in investigations with only a small fraction of time devoted to lecture/discussion. Students collaborated in groups of three to conduct investigations with the use of computer tools and laboratory apparatus. The instructor and teaching assistants interacted directly with the students with the intent to ask probing questions to guide the students through conceptually meaningful problem solving. Researchers video taped students groups as they worked through investigations. Field notes and students' investigation reports provided additional information about student performance. The study reports detailed accounts of student interaction through discourse during the class investigations and comments on the nature of the student collaborations. The study showed that during collaborative problem solving, the students engaged in informal elaborative and reflective discourse that critically examined the data the students had collected during the investigations. The author comments on possible relationships of these interactions and cognitive processes to knowledge construction in an informal setting.