这是出自伊索寓言的。内容如下：
伊索寓言：狐狸和葡萄
一个炎热的夏日，狐狸走过一个果园，他停在一大串熟透而多汁的葡萄前。
狐狸想：“我正口渴呢。”于是他后退了几步，向前一冲，跳起来，却无法够到葡萄。
狐狸后退又试。一次，两次，三次，但是都没有得到葡萄。
狐狸试了一次又一次，都没有成功。最后，他决定放弃，他昂起头,边走边说：“葡萄还没有成熟，我敢肯定它是酸的。”
寓意: 在经历了许多尝试而不能获得成功的时候，有些人往往故意轻视成功，以此来寻求心理安慰。
附原文：
One hot summer day a fox was walking through an orchard. He stopped before a bunch of grapes. They were ripe and juicy.
"I'm just feeling thirsty," he thought. So he backed up a few paces, got a running start, jumped up, but could not reach the grapes.
He walked back. One, two, three, he jumped up again, but still, he missed the grapes.
The fox tried again and again, but never succeeded. At last he decided to give it up.
He walked away with his nose in the air, and said“I am sure they are sour.”
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