Although most instructors would like to believe that their students fully understand every biological concept explained in class, this is often not the case. Gary Wisehart, chair and professor of biology at San Diego City College, knows this from firsthand experience. "Students get very good at telling you what you want to hear," he says, "so it is important to assess the real impact you are having on students' understanding." To do that, Wisehart has been using concept inventories, diagnostic tools designed specifically to uncover lingering misconceptions. This article talks about concept inventory in natural selection (CINS) and discusses how educators are using this tool to correct students' biology misconceptions.