We all know that they do it, but what do students laugh "about" when learning science together? Although research has shown that students do use humor when they learn science, the role of humor in science education has received little attention. In this study, undergraduate students' laughter during collaborative work in physics has been investigated. In order to do this, a framework inspired by conversation analysis has been used. Empirical data was drawn from two video-recorded sessions in which first-year engineering students solved physics problems together. The analysis revealed that the students' use of humor was almost exclusively related to physics. Five themes identified summarize the role of humor in the group discussions:" Something is obvious," "Something is difficult," "Something said might be wrong," "Something is absurd", and "Something said is not within informal norms." This study shows that humor may contribute not only to a good working atmosphere and thereby to the students' learning but also how humor interrelates with both disciplinary culture of physics and its epistemology. The students do not only create and re-create humor that facilitates their social interactions, but through humor they constitute local norms of science and engage with the disciplinary discourse.