「蛙の面に水」（かえるの つらに みず）is like water off a duck’s back.

「蛙の面に水」（かえるの つらに みず） is used to say that something, especially criticism or advice, has no effect on someone.

Isn’t it interesting that the Japanese language uses a frog and the English a duck?

You can also say 「蛙の面に小便」（かえるの つらに しょうべん）. It is to pee on a frog’s face.