Dear Madame L,
You know how, when people want to say that something is going to be impossible, they come up with expressions like "when pigs fly," or "when hell freezes over"?
Why do you think these expressions are so evocative? Do you think there's a difference between the impossibilities we're talking about when we use different ones of these expressions?
Hoping Never to See a Flying Pig
Is that because you don't want to be caught beneath said pig? Or because you don't like the idea of impossible or unexpected things happening?
Great question! When Madame L looked on Wikipedia for "when pigs fly," she found found out that the expression is an "adynaton---a figure of speech so hyperbolic that it describes an impossibility."
At Omniglot you can find out how to say "when pigs fly" in many different languages. Here you'll see that the exact words are often different from those in our English expression. For instance, in Catalan it's "when cows fly," in Spanish it's "when frogs grow hair," and in Danish it's "when hell freezes over."
Which brings us to your second question: Madame L used to think there was a difference. She thought that the images of pigs or cows flying or of grapes growing on willow trees are whimsical and funny, and therefore less ominous than the expression "when hell freezes over." So she thought that hell freezing over would indicate some more dire impossible event.
However, the list of expressions from different languages has disabused her of that notion; and she has searched in vain for a scholarly treatise on the subject.
But Madame L has a great idea: Would any of her Dear Readers like to come up with their own adynatons? These two come to mind immediately: "when hens use toothbrushes," "when the Eagles reunite" (Oh, that one's taken, ironically, if not happily)...
(Please avoid politics and religion.)