Widely used analogies in science textbooks relate unfamiliar phenomena to more familiar everyday objects. However, literature on the illusion of explanatory depth suggests that people are highly overconfident in their understanding of the causal processes underlying how everyday objects and devices work. Thus, such analogies may inflate judgments of understanding (JOUs) for to-be-learned scientific explanations, even when the analogies are presented too superficially to aid understanding. A first experiment showed that superficial analogies notably increase JOUs for geology phenomena when presented and judged as individual sentences. However, a second experiment showed that analogies did not increase the more global JOUs made about the longer segments of the textbook in which the analogies were embedded. The superficial analogies did not help (and sometimes harmed) actual understanding of the geology concepts, suggesting that analogies could sometimes increase the general overconfidence that readers have about their understanding of what they read.