(it's) raining cats and dogs

It's raining cats and dogs, so 

 (like) water off a duck's back

Many people claim that insults or negative comments are like water off a duck's back, but in reality, many of them are

a night owl

Amy is a night owl, so she doesn't usually get up until about

a whale of a time

Ashley said he had a whale of a time at the jazz festival and she

can of worms

The investigators knew that examining the company's books might open a can of worms. They suspected there'd be

drink like a fish

Even though uncle Sam drinks like a fish, he

 zebra crossing

A zebra crossing is a place for

(it's) raining cats and dogs It's raining cats and dogs, so