Sandra Bullock  kicked off her birthday celebrations with a fun-filled night on Thursday. The actress joined pals Chelsea Handler and Whitney Cummings at the Million Dollar Cowboy Bar in Jackson Hole, WY , for a night of dancing, where Sandra cozied up to her sister, Gesine Bullock-Prado, for a cute Instagram snap. But the fun really happened on her actual birthday on Saturday, and things got . . . cheeky. When Sandra and her friends posed for a picture, someone bent over and flashed her thong at the camera. Considering that Whitney is the only one whose face is not seen in the pic and she shared one of herself on the hike, our bet is that she's behind the NSFW mooning. Clearly Sandra shows no signs of slowing down after her 50th bash, and after all of her fun in Hollywood  over the years, it's no surprise that she had such a hilarious celebration.
Source: Instagram user chelseahandler 
Sandra laughed with her sister, Gesine, on Saturday.
Whitney shared a picture of the bar on Thursday.
Chelsea leaned on a boat on Saturday.
Whitney posed on the hike on Saturday.
Everyone posed for a photo after a hike on Saturday.