1:
A is to BasC is to D.
A is to BwhatC 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
