Fostering understanding in students requires taking time to attend to the ideas they already have, both ideas that are incorrect and ideas that can serve as a foundation for subsequent learning (Eaton, Anderson, & Smith, 1984; Minstrell, 1984; McDermott, 1991; Osborne & Freyberg, 1985; Roth, 1991). Such attention requires that teachers be informed about prerequisite ideas/skills needed for understanding a new idea as well as what their students' initial ideas are—in particular, the ideas that may interfere with learning the scientific ideas. Teachers can help address students' ideas if they know what has been shown to be successful (Smith & Anderson, 1983; Roth, 1984; Smith, Blakeslee, & Anderson, 1993). Curriculum materials that alert teachers to their students' probable misconceptions, suggest strategies for identifying and dealing with them, and incorporate appropriate strategies to address students' ideas lead to better student understanding compared with materials that do not (Eaton et al., 1984; Bishop & Anderson, 1990; Lee, Eichinger, Anderson, Berkheimer, & Blakeslee, 1993; Songer & Linn, 1991; Brown & Clement, 1992).

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