The effects of computer animations and mental animation on people's mental models of a mechanical system are examined. In 3 experiments, students learned how a mechanical system works from various instructional treatments including viewing a static diagram of the machine, predicting motion from static diagrams, viewing computer animations, and viewing static and animated diagrams accompanied by verbal commentaries. Although students' understanding of the system was improved by viewing both static and animated diagrams, there was no evidence that animated diagrams led to superior understanding of dynamic processes compared to static diagrams. Comprehension of diagrams was enhanced by asking students questions that required them to predict the behavior of the machine from static diagrams and by providing them with a verbal description of the dynamic processes. This article proposes that predicting motion from static diagrams engages students' mental animation processes, including spatial visualization, and provides them with information about what they do and do not understand about how the machine works. Verbal instruction provides information that is not easily communicated in graphics and directs students' attention to the relevant information in static and animated diagrams. The research suggests that an understanding of students' mental animation abilities is an important component of a theory of learning from external animations.