Its boundary is no longer a pair of circles, but (topologically) a single circle; and what was once its "inside" has merged with its "outside", so that it now has only a single side.
它的边界不再是一对圆圈，而是（拓扑上）单个圆圈；曾经是"内面"的现在和"外面"并了起来，使得它只有"单"面。