They also are effective with U. S. children. In a recent experiment on understanding of mathematical equality, children were randomly assigned to one of three conditions: explain both why correct answers are correct and why incorrect answers are wrong, just explain why correct answers are correct, or just try to solve mathematical equality problems and receive feedback (Siegler, in preparation). Asking children to explain both why correct answers are right and why incorrect answers are wrong led to greater learning than just asking the former type of question. Especially encouraging, it greatly increased transfer to problems that were superficially dissimilar to the originally-presented ones.