Air is to us what water is to fish

1:

A is to B as C is to D.

A is to B what C is to D.

Air is to us as water is to fish。

空气对于我们，就像水对于鱼。此处，what可换为as.

2: 

     the cat is out of bag.泄露秘密。

    ex:so the cat is out of the bag, I want to take you on a vacation.

ex:

You have to promise you won't let the cat out of the bag!

ex:

It's a shame you weren't surprised though, now I know not to tell someone who might let the cat out of the bag next time. .

3:

kicked the bucket :kicked the bucket means she died