3D printers offer extraordinary flexibility for prototyping theshape and mechanical function of objects. We investigatehow 3D models can be modified to facilitate the creation ofinteractive objects that offer dynamic input and output. Weintroduce a general technique for supporting the rapid prototypingof interactivity by removing interior material from3D models to form internal pipes. We describe this new designspace of pipes for interaction design, where variablesinclude openings, path constraints, topologies, and insertedmedia. We then present PipeDream, a tool for routing suchpipes through the interior of 3D models, integrated withina 3D modeling program. We use two distinct routing algorithms.The first has users to define pipes’ terminals, and usespath routing and physics-based simulation to minimize pipebending energy, allowing easy insertion of media post-print.The second allows users to supply a desired internal shapeto which we fit a pipe route: for this we developed a novelgraph-based routing algorithm. We present several prototypescreated using our tool to show its flexibility and potential.