The development of computer environments in which children could inact their operations with number was a critical component of our four year research project on Children's Construction of the Rational Numbers of Arithmetic, funded by the National Science Foundation. These Tools for Interactive Mathematical Activity (TIMA) microworlds continue to eveolve as we refine our models of the children's mathematical constructions. The microworlds are authored as "stand-alone" applications using Object Logo. Using Object Logo permitted us to make refinements in the microworlds in response to the children's actions on a weekly basis. In this presentation I shall demonstrate the latest version of TIMA: Toys, an environment in which children can work with dynamic descrete objects (toys) to create multiple levels of composite units, and use these to construct and operate with multiplicative structures.

*TIMA: BARS (from William K. Bradford Publishing Co.)