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Have you ever heard the expression "when pigs fly?"

It is a saying people use to express that something is very unlikely to happen – as unlikely as pigs growing wings and flying like a bird!

For example, a person who hates riding roller coasters might tell a friend: "I'll go on that ride with you when pigs fly."

Well, one pig in San Francisco may not be flying. But she is helping people relax before they fly in a plane.

LiLou is a small pig working at the San Francisco airport. She is a therapy animal. That means she is calm and likes to be touched by humans. She and other therapy animals aim to help travelers who get nervous about flying in planes.

She was trained by the Society for the Prevention of Cruelty to Animals (SPCA).

This is LiLou's first week on the job as a member of the airport's "wag brigade."

She only works at the airport once a month, so not all travelers to the San Francisco airport will see her. But you might see some of her fellow therapy animals.

She works alongside a group of trained dogs. Their trainers walk around the airport with them. The animals wear signs that say "pet me!"

LiLou the pig has been getting a lot of attention at the airport, and on social media.

One Twitter user said he made fun of the airport's decision to provide a relaxing yoga room for travelers, but he "fully supports LiLou."

LiLou is the first pig to ever to be certified by the SPCA in San Francisco. Jennifer Henley is the manager of the SPCA's program.

"It's wonderful to witness the surprise and delight that LiLou brings to people during therapy visits," she said.

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Dan Friedell wrote this story for Learning English. Ashley Thompson was the editor.