This computer software is a tool designed to help teachers help children in grades 1 through 7 construct their own knowledge of whole numbers and fractions. This software is the first in a series of new, dynamic computer microworlds that allow teachers to interact with students and students to interact with each other as they engage in mathematical activities of reasoning, communicating, and problem solving. Through simple mouse actions, students can make rectangular bars and perform operations using these bars to investigate numerical relationships based on area and measurement concepts. Students can develop strategies for finding sums and differences of fraction quantities, for building equivalent fractions, and for finding fractions of fractions. (MKR)