A common exercise given to students early in a molecular biology course is the creation of a restriction map of a plasmid "digested" by two restriction enzymes (RE). Meanwhile, students have learned from an early age about the properties and analyses of circles in their mathematics courses. But it is rare for students to learn using puzzle-based assignments at the intersection of molecular biology and mathematics. Therefore, we should present students with a puzzle that allows them to combine knowledge and skills from these seemingly disconnected disciplines. Here, we present a method for analyzing RE digests of circular plasmids using basic geometric principles.