From what I remember, water changes back and forth from water in lakes and oceans to vapor, and then back to water again when it falls as rain or snow, as precipitation.

根据我的记忆，水往复地变化，从湖泊和海洋里的水变成水蒸气，然后当它以雨或雪——也就是降水的形式落下时又变成水。

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Comments and expectations about China, especially in the developed world, swung from one extreme end to the other, back and forth.

外界特别是发达国家对中国的评价和期待往往从一个极端到另一极端，落差很大。

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Then they lined up together, each holding a different number, and shuffled back and forth to order their Numbers from largest to smallest, then smallest to largest.

随后他们站成一排，每人举着一个不同的数字，接着来回移动，以按他们手中数字从大到小排序，再从小到大排序。

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That's the finger thing from left finger to right finger going back and forth across the board, never touching a number more than once.

在合并时，我的左手手指,和右手手指来回地在黑板上移动，但对每个数字只会碰到一次。

哈佛公开课 - 计算机科学课程节选

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