curiosity killed the cat  

Meaning: asking questions that may get you in trouble or which might give you information you really don't want to know
How to use in a sentence: When the girl broke up with the boy, he wanted to know why.  She didn't want to tell him that she thought that he was stupid and ugly, so she just said, "Curiosity Killed the Cat."  He wouldn't give up asking her and she finally said that she thought that he was stupid and ugly.  She said to him, "I told you curiosity killed the cat!"

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