"When pigs fly?" Now that just doesn't make sense. When did pigs grow wings? Yet, to a native English speaker, it makes total sense. The phrase when pigs fly is a great example of an idiom, or an expression that has a meaning completely different from that of the words of which it is composed.

When pigs fly

Pig's can't fly! It's impossible, which is precisely what the expression means. For example, you hear someone say, "Do you think Mary will quit her job now that she's pregnant?" Another person responds, "Yeah, when pigs fly! There is no way she is giving up her career!" When he or she used the expression when pigs fly, it means that it's impossible that Mary will quit her job.

When pigs fly

豬不會飛！這是不可能準確表示其意思。例如：你聽過某人說：「Do you think Mary will quit her job now that she's pregnant?」另一個人則回答：「Yeah, when pigs fly! There is no way she is giving up her career!」當他/她使用這個表達方式when pigs fly意思是說Mary要辭去她的職務是不可能的事情。