Some topics are hard to teach, for example explaining how the heart pumps blood. Showing learners short animated movies of a dynamic process can reveal aspects that are too fast to follow, or too small or inaccessible to see. Animations can show how an expert tackles a difficult problem, such as solving a complex equation. They can also show abstractions from the real world, such as the growth of a city. They are useful in stimulating interest and promoting engagement. Learners with special educational needs can benefit from animations that explain an important idea clearly and succinctly, such as how to stay safe online. Learner-created animations are also a way to support self-expression and have been used as prompts for creative activities such as story writing. Some early research studies showed that animations were no better than textbook pictures, but recent research has focused on the conditions that make animations successful as tools for learning. Studies have shown that animations can be better than pictures when they are well-designed, based on sound principles, teach processes or skills, and students are in control.