Background: Self-explanation, or generating explanations to oneself in an attempt to make sense of new information, can promote learning. However, self-explaining takes time, and the learning benefits of this activity need to be rigorously evaluated against alternative uses of this time. Aims: In the current study, we compared the effectiveness of self-explanation prompts to the effectiveness of solving additional practice problems (to equate for time on task) and to solving the same number of problems (to equate for problem-solving experience). Sample: Participants were 69 children in grades 2-4. Methods: Students completed a pre-test, brief intervention session, and a post- and retention test. The intervention focused on solving mathematical equivalence problems such as 3 + 4 + 8 = _ + 8. Students were randomly assigned to one of three intervention conditions: self-explain, additional-practice, or control. Results: Compared to the control condition, self-explanation prompts promoted conceptual and procedural knowledge. Compared to the additional-practice condition, the benefits of self-explanation were more modest and only apparent on some subscales. Conclusions: The findings suggest that self-explanation prompts have some small unique learning benefits, but that greater attention needs to be paid to how much self-explanation offers advantages over alternative uses of time.