What do you think of when you hear the phrase “when pigs fly”? Do funny images of pigs with wings fill your mind? This common saying is used to express the idea that something happening is highly unlikely or even impossible.

For example, if you ask your parents when they’re going to buy you a brand new Ferrari, the answer might very well be: “When pigs fly!” You can spend a lot of your time scanning the skies for flying pork, but your chances of seeing a flying pig are about the same as seeing a shiny new red Ferrari in the driveway.

So why pigs? And why flying? Why couldn’t people just say, “Never. Not a chance!”?

People have used colorful phrases for centuries to get a particular message across. We usually call such phrases “figures of speech.”

These phrases convey a sense of humor and allow people to put a positive spin on things without seeming so negative. Instead of simply saying “no,” someone can communicate the same message in a way that makes it sound — technically — possible.

After all, couldn’t pigs fly one day? granted, their weight and lack of wings weigh heavily against their chances of taking flight. Plus, there’s the whole question of desire. Do pigs have the urge to fly? Without that internal motivation, the seemingly-impossible will probably never become possible.

No one is certain exactly who developed the phrase “when pigs fly.” An old reference to pigs flying appears in Lewis Carroll’s Alice’s Adventures in Wonderland. However, scholars believe Carroll may have picked up the phrase from the Scottish, who allegedly had been using the phrase for a couple hundred years.

They may be right. John Withals’ 1616 English-Latin dictionary lists the proverb “pigs fly with their tails forward.” That similar phrase was apparently used back then as a clever reply to people who made overly-optimistic statements.

There are also those who believe the phrase may have gotten its start in America. Long ago, before the Industrial Revolution, farming was the primary occupation for most Americans. It’s not surprising that many common sayings that developed back then had an animal or farm background.

For example, long ago, farmers used to transport pigs for slaughter along rivers on small barges. If it was foggy, people on shore could only see the pigs’ heads above the fog. This made them look like they were flying through the clouds. This might have given rise to the popular saying.

Other countries also use animals in figures of speech to represent the impossible. For example, many countries use the phrase “when cows fly.” Other countries, such as France and Spain, use phrases like “when frogs grow hair” or “when hens grow teeth.”

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