The Science Museum of Minnesota (SMM) is collaborating with the Museum of Science in Boston (MoS), the North Carolina Museum of Life and Science in Durham (NCMLS), Explora in Albuquerque, the Center for Research in Mathematics and Science Education at San Diego State University (CRMSE), and TERC in Cambridge, MA to develop, create and evaluate "MathCore for Museums," long-term math environments that children can interact with over multiple visits and over several years. The project is prototyping and producing 12 open-source, validated interactive exhibits about proportion: fractions, ratios, similarity, scaling, and percentages, basic concepts for understanding Algebra. The eight best exhibits will be replicated for each MathCore museum and the exhibits will be supported by a limited-access website designed to support and extend repeated use of exhibits and further exploration of ratio and proportion. Selinda Research Associates will conduct a longitudinal evaluation of the project. CRSME will conduct a research study of selected exhibit prototypes to investigate when children start to work on proving relations between similarity and proportion in informal settings, the relationship between children's artwork and mathematical insight, and the roles of bodily activity in learning to see relations in similarity and proportion. Results will be disseminated in peer-reviewed publications, at professional meetings, at the Association of Science and Technology Center's RAP Sessions at the NCMLS, and through the project's website.