Elementary prospective school teachers (PSTs) often struggle to understand why they need to relearn the mathematics that they think they already know. In this set of replication studies, I address this struggle in 3 ways: First, by increasing and varying the participant pool, I replicate Thanheiser's (2009) study, which shows that PSTs do not yet understand number in a way that enables them to teach it. Second, I introduce, validate, and examine the effect of using a survey instead of interviews, thus changing the methods of the original study. Third, I move beyond Thanheiser's original study and show that an interview designed to help PSTs assess their own knowledge accurately correlates with more sophisticated conceptions at the end of the course. Based on these findings, I posit that such an interview could be used to help PSTs learn the mathematics that they need to teach.