Science museum staff face a constructivist dilemma as they design their public spaces: the exhibits should facilitate science learning, yet they also need to support a diverse visiting public in making their own personal choices about where to attend, what to do, and how to interpret their interactions. To be effective as teaching tools, exhibits need to be highly intrinsically motivating at every step of an interaction in order to sustain involvement by an audience who views their visit primarily as a leisure activity. Given these challenges, it is vital to support the design process with a strong program of research and evaluation. I give a personal perspective on one institution's research and evaluation work over the last decade, focusing on four areas: immediate apprehendability, physical interactivity, conceptual coherence, and diversity of learners.