One area explored in the second (in-depth) phase of the Learning in Science Project was "children's science," defined as views of the world and the meanings for words that children have and bring with them to science lessons. The investigation reported focuses on students' (N=39) ideas on the concept of "animal." Data were obtained by the "interview-about-instances" (IAI) procedure in which students were shown diagrams on cards which represented different instances (such as frog) and non-instances (such as fire) of "animal." Additional data were obtained from a 6-question, multiple-choice test, based on the IAI, administered to a larger sample (including high school and college students). Selections from interview transcripts are presented, discussed, and analyzed as they relate to: (1) classification according to biologists' criteria, focusing on the following attributes--number of legs, size, habitat, presence of fur, and noise production; (2) confusion between scientific attributes of living things and those of animals; and (3) use of scientifically acceptable criterial attributes. Selected findings reported indicate that 35 students could not classify all animal instances correctly and (from multiple-choice test results) that 14 percent of Form 1 students, 59 percent of Teachers College students, and 97 percent of university students correctly categorized all six instances. Implications for biology instruction are addressed based on these results. (JN)