Background: Discourse is the teacher's principal pedagogical tool in class. Studies stress the importance of research on discourse in science education and demonstrate its cruciality for understanding science. Purpose: The purpose of this study was to investigate characteristics of the classroom discourse in lessons on physics. Sample: Classroom discourse was studied in five classes at five high schools in Israel, 110 students in all. Altogether, seventeen lessons comprising 373 discourse episodes and 1,892 questions were analyzed. Design and Methods: The discursive characteristics examined were patterns of discourse episodes and their frequency in each lesson, the initiator of discourse (teacher or student), and the types, frequency, and initiators of questions asked. Results: The findings show that despite variance in teachers' and students' traits, all lessons were similar in various parameters of discourse. Most classroom discourse was dynamic and continual; many questions were asked and were of diverse types that induced large numbers of discourse episodes. The lessons differed mainly in the number of teacher-initiated higher-order thinking questions and the number of open discourse episodes -- elements indicative of dialogic teaching. However, even among teachers who asked more high-order questions and initiated open discourse with higher frequency, the discourse episodes were brief and rarely led to meaningful discussions and deep thinking. Conclusions: Investing more in dialogic discourse may be useful in building knowledge and effective learning of abstract ideas in a challenging subject such as physics.