In MicroAquarium, our design approach was to provide free-form and natural interactions for human users, such that the users can engage in creative self-made plays and expressions. Designed activities such as games and cues may enhance user engagement or learning outcomes, but our focus was to provide a naturalistic experience of being in an aquarium. A custom-built light-projection microscope enables interaction with microorganisms. MicroAquarium is designed to support full-body interactions of multiple users. While table-top touchscreen systems may provide prolonged user engagement and more flexible drawing of light stimulation patterns, allowing users to virtually poke on the cells at their fingertips may bring out negative actions and raise ethical concerns. We used gesture sensing to detect users’ hands and implemented mid-air drawing of light patterns, which also overcomes the limited degree of freedom in full-body interactions with shadows. Also, we can overlay the users’ images on the display window of the microscope view, as seeing themselves can help users interact with the system more easily.