This study examines utterances--either words, phrases, or sentences--that instructors initiate and students later appropriate for their own purposes and interests in different sequential contexts. We name these utterances "shared codes" because they occur repeatedly in classroom interactions and gradually take on meanings unique to class members. Simply put, they serve as codes, or "keys," for unlocking the interactional history (but not necessarily for explaining the original meaning). Such shared codes were employed for making humor and building relationships. Indeed, students' use of the codes seems to exhibit learner agency in selecting and appropriating utterances. We examined classroom interactions of two English as a second language (ESL) courses, taught by two instructors in university-based English-language programs in the U.S. Following Cameron and Deignan (2006), this study analyzes interactional contexts where shared codes emerge by employing complex dynamic systems theory perspectives. From our corpus of ESL writing classrooms, we selected four utterances that emerged as shared codes by tracking instances longitudinally, visualizing "patterns" among which the codes emerged. We also employed multimodal conversation analysis to "illuminate" processes during which students adapted their teachers' utterances, thus contributing to those utterances becoming temporarily stable, or in an "attractor state" (Larsen--Freeman, 2017).