Recent research into differences between novice and expert computer programmers has provided evidence that experts know more than novices, and what they know is better organized. The conclusion is only as interesting as it is intuitive. This paper reports an experiment which was designed to determine precisely what novice programmers understand about the behaviour of recursive procedures, and exactly how their understanding differs from an expert's understanding of the process. The results show that different novices understand, or misunderstand, different things. Implications of the findings are discussed with respect to other research into novice and expert programming performance.