There's nothing you can say to upset me - it's water off a duck's back.
That's because ducks have a special coating of grease on their bodies that repels water.
So, someone who doesn't get bothered by criticism can say it's like water off a duck's back.
But hang on, there's another duck idiom - nice/good/perfect weather for ducks.
What would that be then?
Rain, of course - they love it!