The recent decades have witnessed a remarkable increase in the body of research that examine the relationship between humour and language acquisition. This study, however, uses the microanalytical approach of Conversation Analysis (CA) to investigate the impact of the teachers' competent use of the first language (L1) as a source of humour on classroom interaction and, consequently, learning. The turn-by-turn analysis of the data shows how the teacher's use of L1 as a source of humour resulted in smoothness in the interaction and helped in avoiding communication breaking down as a result of the students' insufficient knowledge of the target language (L2) knowledge. The study found that the careful use of L1 as a source of humour also led to the production of longer and more meaningful turns by students, who are characterised as having limited linguistic resources. This use of L1, however, is context sensitive. Pedagogically, despite the scepticism surrounding the use of L1, the paper concludes by emphasising the usefulness of the competent use of L1 as a source of humour in EFL to increase classroom interaction and recommends increasing the teachers' awareness regarding the potential of the competent and occasional use of L1.