What is when pigs fly?

The phrase "when pigs fly" is an idiomatic expression used to describe something that is highly unlikely or impossible to happen. It originates from the humorous idea of pigs, which are ground-dwelling animals without the capability of flight, taking to the skies. The expression is often used sarcastically to dismiss unrealistic expectations or absurd scenarios. Similar expressions exist in other languages, often involving other animals or improbable events, but all serve the same purpose of highlighting improbability.