This study investigated the ability of learner-generated visualizations to improve learning in science. The hypothesis was tested in two domains, a mechanical system and a chemical system, and the results were analyzed separately to compare low and high spatial ability learners. The production of visual explanations of a mechanical system, a bicycle tire pump, increased understanding of the pump particularly for participants with low spatial ability. In the domain of chemical bonding, visual explanations were more effective than verbal explanations for all participants. Visual explanations often included crucial yet invisible features; their accurate construction requires and provides a check for completeness of explanations.