Parents know that the best way to divide a piece of cake between two children is to let one child cut the cake and let the other child choose.
父母们都明白给两个孩子分蛋糕的最佳方法是让一个孩子分，而让另一个孩子选。
As there is no way of deciding which of the two objects is moving, we can choose either one as the moving object.
因为根本没有办法来确定这两个物体中哪一个物体在移动，所以我们可以把两个物体中的任何一个物体认定是在移动着的物体。
And the way it will work is that you have to choose an orientation of a surface, which just means choosing one of the two possible unit normal vectors.
具体方法是，先给曲面定向，也就是选择两个单位法向量中的一个。
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