In two experiments, we investigated how learners comprehend the functioning of a three-pulley system from a presentation on a computer screen. In the first experiment (N = 62) we tested the effect of static vs. animated presentations on comprehension. In the second experiment (N = 45), we tested the effect of user-control of an animated presentation on comprehension. In both experiments the participants were university students. Comprehension was measured with a test including three comprehension indicators. The first experiment indicated that an animation as well as integrated sequential static frames enhanced comprehension. The second experiment showed that a controllable animation did not have a powerful effect on comprehension, except for learners with low spatial and mechanical reasoning abilities. (Contains 4 tables and 2 figures.)