There are many commercial toys and learning products meant to help K-12 students learn about robotics and electronics, but nothing similar exists to excite students about fluid mechanics and heat transfer. Consequently, many students who might grow up to work in these fields never even learn what they entail. In addition, while robotics and electronics are engaging for many students, water and heat design challenges may provide entry into engineering for students with different interests. With this in mind, a modular, open-ended toolkit for building fluid mechanics and heat transfer experiments is being developed for upper elementary and middle school students. The kit consists of tubes, valves, junctions, water tanks, and heaters that can be connected in any order to build flow and heat transfer devices. Preliminary tests have been conducted to evaluate the usability and functionality of the kit by asking third through seventh grade students to build plant watering and house water supplying systems. The tests indicated that the FlowGo toolset is easy to use, has a high level of engagement, and shows the potential to engage students in design challenges related to gravity-driven flow and heat loss through pipes. Background and Motivation