for advice.Experiments showed that ten-year old children are able to create algorithms, i.e., sequences of operations that solve problems, and that their gestures help them to do so. The theory of mental models, which is implemented in a computer program, postulates that the creation of algorithms depends on kinematic mental simulations. Gestures are outward signs of moves and they help the process. We tested ten-year children, because they can make mental simulations, and because they gesture more than adults. They readily rearranged the order of six cars in a train (using a siding), and the difficulty of the task depended on the number of moves in minimal solutions (Experiment 1). They were also able to devise informal algorithms to rearrange the order of cars when they were not allowed to move the cars, and the difficulty of the task depended on the complexity of the algorithms (Experiment 2). When children were unable to gesture as they formulated algorithms, the accuracy of their algorithms declined by13% (Experiment 3). We discuss the implications of these results for accounts of reasoning and theories of gestures.