The 18-year-old jokingly sneaks up behind her big sister during a workout

By Kathy Ehrich Dowd
Updated January 11, 2014 01:40 PM
Credit: Courtesy of Kendall Jenner

That is one booty-ful photobomb.

Kendall Jenner jokingly showed off her, um, assets Friday when she sidled up next to big sister Kim Kardashian‘s famous derriére.

“Barging in on Kim’s booty selfie,” Jenner, 18, captioned the Instagram shot she posted early Friday.

The duo certainly look ready for a workout: The sisters are both sporting figure-hugging black workout pants and sports bras. Kardashian, 33, is also wearing black Nike sneakers, while the flash blocks out her face.

Booty-flaunting selfies and grueling workouts are nothing new for Kardashian, who has been working on her post-baby body ever since welcoming daughter North with fiancé Kanye West in June.

She also recently made it clear that her hard work at the gym is responsible for the return of her famous figure – and nothing else.

“It sucks when people make up surgery or photoshop lies when I am so disciplined,” she wrote on Twitter Friday.

Also not new for the reality star? Appearing in amusing photos with family and friends, as evidenced by this sweetly dorky Throwback Thursday photo released this week, featuring Kim and pal Nicole Richie at 13.