Students are said to be at an "impasse" during problem solving when they either get stuck and do not know what to do next, or have two or more choices and don't know which one to take, or have discovered that they have made an error and do not know how to fix it. When studying an example and trying to self-explanation it, student can reach an impasse when they don't know how or why to a step was derived--they can't explain it. When working with a tutoring system that gives immediate feedback after each step, as the cognitive tutors and Andes do, students can reach an impasse when the system indicates that a step is wrong and they don't know how to fix it.
Impasses have played an important role in theories of cognitive skill acquisition, in part because they are thought to motivate students to seek knowledge and repair their incorrect or missing knowledge components. See
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