Adults overestimate the detail and depth of their explanatory knowledge, but through providing explanations they recognize their initial illusion of understanding. By contrast, they are much more accurate in making self-assessments for other kinds of knowledge, such as for procedures, narratives, and facts. Two studies examined this "illusion of explanatory depth" with 48 children each in grades K, 2, and 4, and also explored adults' ratings of the children's explanations. Children judged their understanding of mechanical devices (Study 1) and procedures (Study 2). Second and fourth graders showed a clear illusion of explanatory depth for devices, recognizing the inaccuracy of their initial impressions after providing explanations. The illusion did not occur for knowledge of procedures.