New technologies allow the display of text, static visuals, and animations. Although animations are inherently attractive, they are not always beneficial for learning. Problems may arise especially when animations modify the learner's cognitive load in an unintended way. In two learning experiments with 40 and 26 university students, the effects of animated pictures on knowledge acquisition were investigated. Some pictures displayed visual simulations of changes over time, whereas other pictures could be manipulated by learners to represent different states in time. Results showed that manipulation pictures had an enabling function for individuals with high learning prerequisites, whereas simulation pictures had a facilitating function for individuals with low learning prerequisites. However, the facilitating function was not beneficial for learning, because learners were prevented from performing relevant cognitive processes on their own. A careful analysis of the interrelation between different kinds of cognitive load and the process of learning is therefore required.