Thursday (July 22) happens to be Selena Gomez's 18th birthday, and while having her first feature film, [article id="1644103"]"Ramona and Beezus,"[/article] hitting the big screen this weekend is quite a gift -- and she got a giant cupcake backstage at "Late Night With Jimmy Fallon" Wednesday night -- Gomez has devised a perfect way to spend this milestone birthday.

"[I'm going to have a] barbecue with my friends," Gomez told MTV News at the junket for the film. "['Ramona and Beezus' co-star] Joey [King] is coming. We're gonna go swimming."

As excited as King is for the chance to chill with her onscreen big sister, accepting the invitation comes with one major stipulation. "She finally took the fiberglass out of the pool," King explained about an earlier pool party. "I slept over the other time and there was fiberglass in it!"

Gomez never explained how it got there, but she did point out that King doesn't have anything to worry about this time around. "All of a sudden [King] starts screaming in my pool, like, 'Why is it burning?!' " Gomez recalled. "We took it out, so this time it'll be good. I promise."

The 18-year-old has been candid about her kinship to her 10-year-old co-star, pointing out that she really has become like a little sister to her.

"Even when they stopped, when they would say 'Cut,' we would still just be as loud and talking. We all got along really well. We talk all the time," Gomez explained. "She's like my sister, and I never had a sibling, so she's as close as I've got. She's still the Ramona. I go to her for advice."

Last weekend, Gomez kicked off the party early, tweeting that she had some early birthday plans. She wrote, "Went to dinner with my family and some fans sang happy birthday to me! What an awesome early birthday gift :) Thank you."

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