Background: Learning-by-teaching is a generative learning strategy in which students are told they will have to teach what they are learning to others. Although learning-by-teaching has been shown to be effective in some cases, few studies have established guidelines for how to optimize the benefits of learning-by-teaching as a generative learning strategy from a social presence perspective. Objectives: This study seeks to clarify the learning-by-teaching hypothesis and to pinpoint the optimal level of social presence during learning-by-teaching that is most conducive to learning. Methods: In Experiment 1, college students received a lesson with instructions that afterwards they would explain the material to others by making a video, explain the material aloud to themselves, or restudy the material. In Experiment 2, college students viewed a multimedia lesson with instructions that afterwards they would explain the materials by making a video, explain to an onscreen student, or explain to a student in person. Results and Conclusions: Teaching by making a video was better than restudying, self-explaining, and teaching face-to-face or online. Teaching quality was better in video teaching than self-explaining and face-to-face or online teaching. Teaching by making a video is ideal because it primes generative processing while minimizing extraneous processing. Implications: This study is the first to manipulate different levels of social presence of oral teaching to determine the optimal form of learning-by-teaching, which preliminarily clarifies generative learning and social presence theory and has implications for both empirical and theoretical research.