Cónɡqián , yǒu yíɡèQǐɡuórén fēichánɡdǎnxiǎo , érqiějīnɡchánɡyǒu yìxiēqíɡuài de xiánɡfǎ.Yìtiān wǎnfàn hòu , tāzuòzài ménqián chénɡliánɡ , tūrán xiǎnɡdào : " rúɡuǒyǒu yìtiān tiān tāxiàlái , wǒmen búshìjiùyào bèi yāsǐle ma ? "Cónɡcǐyǐhòu , tāměitiān dōu wéi zhèɡe wèntífánnǎo . pénɡyou men hěn dānxīn tā , quàn tāxiǎnɡkāi diǎn , tiān búhuìtāxiàlái , jiùsuàn zhēnyào tāxiàlái , yěbúshìtāyíɡèrén kéyǐjiějuéde . Kěshìbúlùn biérén zěnme shuō , tāháishìchánɡwéi zhèɡe búbìyào de wèntídānxīn .
从前，有一个杞国人非常胆小，而且经常有一些奇怪的想法。一天晚饭后，他坐在门前乘凉，突然想到：“如果有一天天塌下来，我们不是就要被压死了吗？”从此以后，他每天都为这个问题烦恼。朋友们很担心他，劝他想开点，天不会塌下来，就算真要塌下来，也不是他一个人可以解决的。可是不论别人怎么说，他还是常为这个不必要的问题担心。
Once upon a time, a man lived in the state of Qi was really timid and often got absurd ideas. One day after dinner, he went to relax at his doorway. Suddenly, he thought, "If one day, the sky falls down. We will be all crushed to death." From then on, he was bothered by this thought constantly. His friends really worried about him, and suggested him to be more open-minded. They talked to him that the sky wouldn't fall down and even if it would, it would not be a problem he can solve. However, no matter what was said to him, he still worried about this unnecessary problem.

Explanation
胆小dǎnxiǎo：timid（ adj. ）
乘凉chénɡliánɡ：to relax in a cool place（v.）
塌tā： collapse（v.）
从此cónɡcǐ：from then on（conj.）
烦恼fánnǎo：to bother about（ v.）
劝quàn：persuade （v.）
解决jiějué：solve（v. ）
不必要bú bìyào：unnecessary（adj. ）

Meaning

“杞”是古代地名，“忧”的意思是“担忧”、“焦虑”。“杞人忧天”的意思是杞人担心天会塌下来，比喻缺乏根据或不必要的担心和忧虑。
"杞(Qǐ)" is a name for an ancient place. "忧(yōu)" means worry. “杞人忧天(Qǐrén Yōu Tiān)” means that a man lived in Qi worried about the sky would fall down. It refers to unnecessary or groundless worries and anxieties.

Sentences

1.  Tāměicìkǎoshìchénɡjìdōu fēichánɡhǎo , kětāfùmǔháishìdānxīn tāhuìkǎo búshànɡdàxué , wǒkàn zhèjiǎnzhíshìqǐ rén yōu tiān .
他每次考试成绩都非常好，可他父母还是担心他会考不上大学，我看这简直是杞人忧天。
He always did a good job at every examination, but his parents still worried that he might not be able to pass the college entrance exam. I think they were completely 杞人忧天.

2.  Kēxuéjiāmen rènwéi shìjièmòrìde shuōfǎbìnɡbúshìqǐrén yōu tiān , rúɡuǒbúzhònɡshìhuánjìnɡbǎohù , shēnɡtài jìxù èhuà , shìjièmòrìzhēnde huìjiànɡlín .
科学家们认为世界末日的说法并不是杞人忧天，如果不重视环境保护，生态继续恶化，世界末日真的会降临。
Scientists agree that the doomsday is not entirely 杞人忧天. If we don't pay attention to environmental protection, the ecological system will become worse and worse and the end of the world will finally come. 

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