瞎猫逮住死耗子(xia mao daizhu si haozi)
Usually, a blind cat cannot catch a rat. The saying "a blind cat stumbled on a dead rat" means "something happened by sheer luck." Sometimes, people use the saying to show their modesty, to express "as luck would have it, they have fulfilled some difficult task."
Example: A: Could you tell me your secret for winning the lottery?
B: There's no secret. It's just sheer luck.
