Apparently lots of people learn the phrase "it's raining cats

and dogs" from English textbooks. If you say this, English speakers will understand what you mean, but they never really say it themselves - it sounds quite weird.

I'm not sure why this is taught so often - something like "it's chucking it down" is better - it's quite a common, informal way of saying it's raining heavily in the UK (I'm not sure about the US)