Few laboratory exercises are designed to teach biology students about barriers that may constrain the movement of organisms. We describe a unique inquiry-based exercise involving Lego mazes (the barrier) and the plasmodial slime mold, Physarum polycephalum (the organism). During guided inquiry, students con- struct mazes using Lego brand building blocks and the slime mold is allowed to "navigate" through the maze and "respond" to the barrier. Students then generate and test hypotheses about the movement of the slime mold in response to different barriers in the open-inquiry phase of the investigation.