Computers possess a unique range of properties to assist the learning of Mathematics and Science. Of particular importance is their interactivity, exemplified by supporting Direct Manipulation, Memory Augmentation, Qualitative Reasoning, Conflict Resolution and the presentation of Counter-Factual examples. This paper describes two ways in which these properties have been explored. Firstly, in the design of an ‘alternate realities´ simulation for exploring horizontal motion. Secondly, by focusing on the importance of internal and self-generated external visual representations in problem solving, the design of a combined animation / modelling system is suggested.