The rise of biodiversity-related citizen science projects gives us the opportunity to inspire all students to become citizen scientists, no matter what their professional path. Here, I share an in-class exercise that introduces undergraduates to biodiversity and citizen science using the open-source platform iNaturalist. I developed the exercise as part of a semester-long module on bumblebee diversity and conservation, embedded in an introductory environmental science course for majors and non-majors. Bumblebees worked well in the context of this course because they are easily recognized, charismatic organisms that serve important ecological roles and face a variety of threats. The objectives of the iNaturalist bumblebee module include (1) introducing students to bumblebees at global, regional, and local scales; (2) using a citizen science data base to predict what species are likely to be found in specific geographic regions; and (3) articulating the power and limitations of citizen science data bases. While this exercise focuses on bumblebees in North America and the state of Missouri, where my university is located, iNaturalist is a flexible platform that can be adapted to different geographic regions. It can also be used for on-line courses. I describe how I implemented this activity as a 50-minute in-class exercise and follow-up discussion.