Microscopes play a central role in hands-on science exploration and communication. All too often, however, students do not have a good understanding of the optical principles that govern microscopy and lack access to instruments that could help them build that understanding. Here we present an open-source Do-It-Yourself (DIY) microscope kit developed by scientists and students at BioBus, a nonprofit organization for science outreach and education based in New York City (www.biobus.org). The DIY microscope uses 3D printing to make highly adaptable optical technology readily available to educators at a low cost. Its modular configuration makes it the perfect tool to teach optical design to students of all ages. At BioBus, Inc., over 230 school-age students and teachers learned basic optics and microscope building with the DIY microscope at our community and mobile laboratories. We further present examples of how the DIY microscope was used as a platform for student-driven projects, expanding the original design to include advanced optical features such as fluorescence and infrared imaging. The images, acquired with a low cost camera, were comparable in quality to those taken using professional grade laboratory microscopes. The use of the DIY microscope is not limited to applications in physical sciences, but can also be used as an interdisciplinary teaching tool. As an example, we showed how it was configured into a functional model of the eye, to explain the physics of vision and the pathophysiology of eye disorders, such as far and short-sightedness, and age-related macular degeneration.