Learning-by-teaching is a generative learning strategy in which students are asked to teach what they are learning to others (Fiorella & Mayer, 2015). In this study, college students watched a multimedia lesson on chemical synaptic transmission with instructions that afterward they would explain the materials by making a lecture video (teach-to-camera condition), explain to a student face-to-face (teach-to-student condition), or explain to seven students face-to-face (teach-to-group condition), and then they engaged in the corresponding teaching activity, respectively. Compared to the other two groups, the teach-to-camera condition performed significantly better on a transfer test, reported significantly lower social presence, experienced significantly lower arousal as measured by pulse rate, and engaged in significantly more generative processing via quality explanations, which were measured by a number of idea units, elaboration statements, and monitoring statements. The teach-to-camera condition significantly outperformed the teach-to-group condition on a retention test and reported significantly lower state anxiety, teaching difficulty, and cognitive load during teaching than the teach-to-group condition. The relation between audience presence and learning outcome was mediated by the negative impacts of distraction during teaching (e.g., anxiety or extraneous cognitive load ratings) and the positive impacts of generative processing during teaching (e.g., number of idea units generated). Teaching to an imaginary audience via video is more effective for learning than teaching to a student or group of students because it minimizes distraction while freeing up the capacity to increase active generative processing.