Teaching educational materials to others enhances the teacher's own learning of those to-be-taught materials, although the underlying mechanisms remain largely unknown. Here, we show that the learning-by-teaching benefit is possibly a retrieval benefit. Learners (a) solved arithmetic problems (i.e., they neither taught nor retrieved; "control" group), (b) taught without relying on teaching notes (i.e., they had to retrieve the materials while teaching; "teaching" group), (c) taught with teaching notes (i.e., they did not retrieve the materials while teaching; "teaching without retrieval practice" ["TnRP"] group), or (d) retrieved (i.e., they did "not" teach but only practised retrieving; "retrieval practice" group). In a final comprehension test 1 week later, learners in the teaching group, as did those in the retrieval practice group, outperformed learners in the TnRP and control groups. Retrieval practice possibly causes the learning benefits of teaching.