“that cat won't jump”别理解成“那只猫不会跳”，而是那个想法行不通，那个建议不现实。

【例句】
I hear many saying that we ought to do away with taxes altogether, but I'm afraid that cat won't jump.
我听到很多人说，我们应该完全取消税收，但我觉得那个想法行不通。

Going to cramming school before the exam is not realistic. That cat won't jump.
考试前去补习班是不现实的，那个想法行不通。