Here's why:
* Meaning: It doesn't literally mean that pigs will be flying in the sky. Instead, it conveys the idea that something is completely impossible or highly unlikely to happen.
* Figurative Language: It uses a vivid image (flying pigs) to express a metaphorical meaning.
* Common Usage: It's a widely understood and commonly used expression in English.
So, while it might sound like a simple statement, "when pigs fly" actually carries a deeper, figurative meaning.