Confidence is an important issue for biology students in handling computational concepts. This paper describes a practical in which honours-level bioscience students simulate complex animal behaviour using StarLogo TNG, a freely-available graphical programming environment. The practical consists of two sessions, the first of which guides students through building their own computer simulation using StarLogo TNG's graphical programming blocks. The second practical requires them to modify the simulation and carry out a simulation-based experiment. Results from pre- and post-surveys show that, after completing the practical, students have increased their confidence in answering questions requiring understanding of computer code. (Contains 5 figures.)