What's the catch?

You can say this when something sounds too good to be true. It means that you suspect that there is a hidden problem, which might make the opportunity not as good as it appears to be.

当某事听起来好得令人难以置信时，你可以这样说。这意味着你怀疑有一个隐藏的问题，这可能使事情不像它看起来那么好。

当老外说What's the catch?它表达的意思就是，我已经知道所有好的方面了，你能不能告诉我它潜在的风险(risks)或者是潜在的问题(hidden problems)。

A: I just found out about a great opportunity I can work from home only 5 hours a week and I ll earn at least $2000 a month!

我刚刚发现了一个很好的机会，我可以每周在家工作5个小时，每月至少能挣2000美元!

B:What's the catch?

这其中有什么猫腻?

(you think that there is some problem or lie about this opportunity)

(你认为这个机会是个骗局或者会有其他问题)