Celebrating hands-on making and technological inventiveness, the Maker Movement promotes the popularity of new makerspaces: learning environments filled with diverse materials for youth's creative projects. Described as "constructionist learning environments," makerspaces can be challenging to design because materials require substantial budgetary investments. Because the practical demands of space and cost often dominate decisions concerning a new makerspace, less attention is paid to how the choice of materials inadvertently limits who uses the space, how learning happens, or how materials in a space will interact and intra-act over time. Building on theories of constructionism and relational materialism to analyze and theorize learning in makerspaces, we take a case-based approach to illustrate the co-development of 3D printing materials, youth and educational programs at an out-of-school makerspace. In the process, we demonstrate the need to rethink the role of materials in human development and educational design. We introduce the concept of "materials-to-develop-wit"h to explain how objects can be internalized "and" drive the development of spaces, people and learning opportunities.