Mechanistic reasoning is typically contrasted with teleological reasoning in biology education. Whereas mechanistic reasoning plays an important role in building scientific explanations of natural phenomena, students’ teleological reasoning is considered a major learning obstacle. In this paper, we characterize students’ teleological reasoning as an explanatory escape in their search for mechanistic explanations. When attempting to build valid explanations, students struggle to construct step-by-step chains from cause to effect. If students lack knowledge about an explanatory step, they try to use existing knowledge to predict the involved entities and activities. When confronted with knowledge gaps, students take recourse to the most salient information available to them, the function of biological phenomena, and hence they reason teleologically. This argument is based on a review of the literature on the philosophy of science and cognitive psychology, as well as data from personal interviews with 23 students using three consecutive tasks on the evolution of the giraffe’s neck. Our data show that teleological reasoning correlates with a lack of knowledge about evolutionary key concepts. Students who lack evolutionary knowledge base their reasoning on knowledge elements from their everyday life and information introduced in the interview task. We argue that teachers should guide students’ reasoning by deliberately introducing information in tasks and that students need to acquire strategies to guide their own search for mechanistic explanations.