rack your brains 

UK (US rack your brain)

rack /ræk/ v. 變形、折磨；榨取

to think very hard

絞盡腦汁，搜腸刮肚

I've been racking my brains all day but I can't remember her name.

我搜腸刮肚想了一整天，卻怎么也記不起她的名字來。

Don't think this job a piece of cake. You'll have to rack your brains once you start on it. 

別以為這工作是易如反掌的事，你做了就知道它是很傷腦筋的。

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