The problem of hunger worldwide is a hard nut to crack. I'm afraid that scientists need to put in more efforts.

全球性的饥饿问题是个棘手的问题，恐怕科学家们需要付出更多的努力。

Each has his own hard nut to crack.; Each has his own trouble.

各有一本难念的经。

A hard nut to crack and find!

一种难以寻找而且难以对付的坚果。

Sudan was a hard nut to crack.

苏丹真的是难啃的骨头。

I know he, he is a hard nut.

我认识他，他是个感恩不化的人。