Understanding what children know is paramount to planning effective science instruction; however, in any classroom, the students hold a variety of ideas. Representing these differences in ways that also acknowledge the common trends among students might facilitate the process of differentiation. To exemplify one such possible process of representing students' ideas, we used to the statistical method of latent class analysis to understand 480 students' (Grades 1-8) responses to the question, "What is a day?" The initial coding of students' drawn and written responses allowed us to identify a variety of ideas drawn on by the students. For example, younger children provided very personal responses and older students provided more scientific or mathematical responses. The subsequent quantitative analysis revealed ways that ideas were connected in groups of students, ultimately providing descriptions of progressively sophisticated responses typical at each grade level. (Contains 2 tables and 11 figures.)