Gwen Stefani and surprise guests join James Corden for Carpool Karaoke

Gwen Stefani has joined a celebrity rite of passage: James Corden's Carpool Karaoke. The singer joined the lovable Brit presenter to belt out some of her greatest hits and discuss the ideas behind her latest album, with some help from a few surprise guests along the way.

James, 37, kicked things off with No Doubt classic "Don't Speak," which he and the 46-year-old sang with ease.

Gwen Stefani is amazing in the latest Carpool Karaoke with James Corden.

"Don't Speak" was released 20 years ago, so it's no surprise that James took the opportunity to quiz mother-of-three Gwen on how she manages to stay so youthful, nicknaming her Benjamin Button.

But Gwen had the perfect answer: the "Cool"hitmaker credited her new relationship with country music singer Blake Shelton, insisting their romance has given her a facelift!

Gwen and James were joined by George Clooney and Julia Roberts!

During the journey, they worked through tracks including "Sweet Escape" – with Gwen helping James out with the lyrics – and she revealed her obsession with emojis. But the real surprise came when they entered a HOV-4 lane, meaning the car needed another two occupants.

Enter George Clooney and Julia Roberts! The Hollywood actors kindly helped James and Gwen out of their situation, and George treated everyone to an amazing rendition of "Hollaback Girl" – which almost beats Jennifer Lopez' prank texts to Leonardo DiCaprio

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Discussing Gwen's feisty solo hit, James quizzed her on the inspiration behind the words. While George confessed he thought it was about a girl ignoring wolf-whistles from a building site, Gwen teased that she will never disclose the real reason she wrote the song.

"Just know that I won," she revealed mysteriously, leaving James – and all of us – desperate to know more.

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