Miley also revealed that she used to pick on Baldwin when they were kids.
Kendall and Baldwin rocked out to popular songs including Panic! At The Disco's "I Write Sins Not Tragedies" and the Bill Withers classic "Lean On Me." But when the BFFs were jamming to Miley's 2009 hit, "Party in the U.S.A.," the freshly minted Mrs. Bieber noticed a blonde figure dancing outside the car window.
When the chorus began, Miley popped up again but this time in the back seat and joined in on singing her classic song, which let's be real, is still a bop after all these years. The three girls nodded their head like yeah, moved their hips like yeah and had a party in the U.S.A.
"Dude, my inner 15-year-old is freaking out," Kendall admitted after the song ended and Baldwin agreed.
Watch an adorable clip from the show below.
Baldwin, 22, who married singer Justin Bieber, 24, back in September, revealed that she and Miley, 26, have been friends for years. (Who knew?!) The "Wrecking Ball" singer explained that she and Baldwin's older sister, Alaia, used to pick on Baldwin when she was a kid.
"I would be evil to her," Miley said. "She would try to play with us -- like me and Alaia [Baldwin]-- and we would like lock her out of the room."
"My older sister and her used to gang up on me!" Baldwin said and Miley added, "Now you can take all the trolls and shit, because I was such a troll."
Baldwin pointed out that, Miley, who has a lot of experience being in the public eye, "prepared" her for the industry.
The party continued as Miley, Baldwin and Kendall bopped to Miley's new single "Nothing Breaks Like A Heart."
Of course Kendall couldn't help but bring up the big elephant in the room -- Baldwin's relationship with Bieber. If you recall, the "Carpool Karaoke: The Series" trailer released in October, teased the episode and featured Baldwin and Kendall participating in a best friend lie detector test, where the then newly-engaged Baldwin had to be honest whether or not Bieber thought Kendall was "cool."
Baldwin and Kendall also danced to Bieber's hit "Company," performed their secret handshake and changed vehicles a few times, including jumping into a monster truck.
You can watch the entire episode now on Apple TV.
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