Engaging in explanation, even to oneself, can enhance learning. What underlies this effect? Williams & Lombrozo (in press) propose that explanation exerts subsumptive constraints on processing, driving learners to discover underlying patterns. A category-learning experiment demonstrates that explanation can enhance or impair learning depending on whether these constraints match the structure of the material being learned. Explaining can help learning when reliable patterns are present, but actually impairs learning when patterns are misleading. This explanation impairment effect is predicted by the subsumptive constraints account, but challenges alternative hypotheses according to which explaining helps learning by increasing task engagement through motivation, attention, or processing time. The findings have both theoretical and practical implications for learning and education.