WHEN PIGS FLY

Is it possible for a pig to fly? Have you ever seen a pig fly? 
A person uses the idiom - When Pigs Fly - to express, with sarcasm, that something will never happen or that they will never do something. 


For example: 


A:"Will you come see the new Star Trek movie with me?"
B: "When pigs fly! I hate science fiction."

(Image courtesy of photosearch stock illustration)