人心不足蛇吞象 The heart of man is not content, the snake swallows the elephant
Explanation
这个成语比喻人贪得无厌，不知足，就像蛇想要吞食大象一样，根本不可能实现。
This idiom describes a person who is insatiable and greedy, never satisfied, like a snake trying to swallow an elephant, which is impossible.
Origin Story
古时候，有一条小蛇，它非常贪心，总是想吃更多的东西。一天，它看到了一头大象，就想要吞下它。它努力地张开嘴巴，拼命地往里塞，可是一点也塞不进去。最后，它累得筋疲力尽，不得不放弃了。这个故事告诉我们，人要懂得知足，不要贪心不足。就像那条小蛇，它想要吞下大象，但它最终还是失败了。
Once upon a time, there was a little snake that was very greedy and always wanted to eat more. One day, it saw a big elephant and wanted to swallow it. It tried its best to open its mouth and stuff the elephant in, but it couldn't get it in at all. Finally, it was exhausted and had to give up. This story teaches us that we should be content and not be greedy. Like the little snake, it wanted to swallow the elephant, but it failed in the end.
Usage
这个成语通常用来形容人贪得无厌，不知足，也用来讽刺那些不切实际，痴心妄想的人。
This idiom is often used to describe people who are insatiable and greedy, or to ridicule those who are unrealistic and daydreamers.
Examples
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他这个人～，总是想要得到更多，殊不知过犹不及。
ta zhe ge ren ren xin bu zu she tun xiang, zong shi xiang yao de dao geng duo, shu bu zhi guo you bu ji.
He is so greedy, he always wants more, not knowing that too much is as bad as too little.
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你以为你能得到天下，却不知～，最终只会得不偿失。
ni yi wei ni neng de dao tian xia, que bu zhi ren xin bu zu she tun xiang, zui zhong zhi hui de bu chang shi.
You think you can conquer the world, but you don't know that you will lose more than you gain in the end.
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～，不要总是贪婪，要懂得知足。
ren xin bu zu she tun xiang, bu yao zong shi tan lan, yao dong de zhi zu.
Don't always be greedy, learn to be content with what you have.