It is the child's thinking that has not turned a corner, because addition belongs to positive thinking (that is, sequential thinking) and subtraction belongs to reverse thinking (reverse thinking). In our daily life, the most contact and deepest experience is positive thinking. When children are studying, it is easier to accept the addition from small to large, but the subtraction from large to small is not easy to turn a corner. Therefore, children should be taught to think in reverse and do more problems, such as addition.