Many researchers have noted that students' whole number knowledge can interfere with their efforts to learn fractions. In this paper the authors discuss a teaching experiment conducted with students in Years 5 and 6 from an eastern suburban school in Melbourne. The focus of the teaching experiment was to use number lines to highlight students' understanding of whole numbers then fractions. The study used a screening test and interview using number lines to probe students' understanding of fractions as numbers. By using whole numbers on number lines first, the interview questions clearly helped many students to connect whole number and fraction knowledge. The interviews also helped students to recognise and correct their own misconceptions in previous assessment tasks. This research showed that successful students had easily accessible whole number knowledge and recognised the relationship between the whole and the parts whereas the weakest students had poor number knowledge and could not see the connections. [Author abstract, ed]