Actually for ESL
We say "when pigs fly" when talking about situations that will never happen
What might you say to someone when you tell them not to worry?
What can you say when someone/something annoys you?
What can you say to someone is being unusually quiet?
What do you say when someone is totally wrong about their course of action?
When someone is really nervous, they are...
A fish out of water is someone is out of their usual and comfortable environment
In an impossible situation you could say...