rack one's brain 绞尽脑汁，努力思索

　　Write them down and then you won't have to rack your brain anymore!

　　把这些都写下来，你就不用每次都拼命想了。

　　The test is too hard. I have been racking my brain for days.

　　这个测试太难了，我已经绞尽脑汁想了几天了。