Idioms/Phrase for Obvious No

There are idioms for an obvious yes when answering a question maybe if you were offered something and replying for example : 'Does the pope pray?' And various other ones.

What are the equivalents for a no?

Solution 1:

Consider,

When hell freezes over Ngram

until hell freezes over

Forever I will fight for these children until hell freezes over. Usage notes: also used in the form when hell freezes over (never): They said they would get back together when hell freezes over. Etymology: based on the idea that hell (a very hot place where people are punished forever) can never freeze Cambridge Dictionary of American Idioms

When Burnham Wood comes to Dunsinane --Shakespeare. WordReference

Sure he'll help me wash the dishes...when Burnham Wood comes to Dunsinane.