打破砂锅问到底/ 打破砂鍋問到底 dǎpòshāguōwèndàodǐ to get to the bottom of things
To insist on getting a detailed answer or finding out the root cause of something. This idiom is a pun: a broken clay pot has a crack that goes to the bottom, and the word for crack 璺 wèn sounds like the word for ask 问 wèn.