This study examined siblings' knowledge about the teaching concept during naturalistic teaching contexts, wherein children's communicative interactions were used as a gateway to their social understanding (Turnbull, Carpendale, & Racine, 2009). Participants included 39 sibling dyads (older age group, M[subscript age] = 6;4; younger age group, M[subscript age] = 4;5) observed for six 90-min sessions at home. Teaching episodes were identified and coded for: a) initiation of teaching (i.e., assumes role or learner requests teaching), b) knowledge states (i.e., knowledge, lack of knowledge, questioning knowledge), c) transfer of knowledge (i.e., learning), and d) teaching strategies (e.g., direct instruction). Children who assumed the teaching role referenced knowledge and questioning knowledge, whereas learners requested teaching by referencing a lack of knowledge. Firstborn learners were more likely to reference knowledge versus second-born learners who referenced a lack of knowledge. Transfer of knowledge occurred when teachers referenced knowledge. When learners referenced knowledge states, teachers were more likely to use direct instruction, demonstration, and negative feedback. Results underscore the developmental significance of sibling teaching and demonstrate a novel approach to studying children's social understanding in the teaching context (Turnbull et al., 2009).