Justin Bieber, Ke$ha Get Birthday Party Advice From '21 And Over' Stars

'When Justin Bieber and Ke$ha go to a movie together ... you officially enter the fifth element,' Skylar Astin jokes to MTV News.

What is Justin Bieber to do with a night off from his Believe Tour and a birthday to celebrate? Well, celebrate, obviously. And, who better to offer up some birthday party advice than the stars of "21 and Over," a movie all about living it up on your b-day.

Skylar Astin told MTV News what decor he'd use for the fete: "Some old-school balloons, maybe some silly string. I don't know if there'll be enough time."

"You've got to have some bouncy castles," Miles Teller added.

These party planners had even more ideas up their sleeves. One might say it's their pop secret. "Popcorn stand, cotton candy machine," Justin Chon suggested, as Astin added, "I want different colored popcorn too, remember how they used to do that?"

Keeping in the tradition of their "Hangover"-type movie, Teller just really wants to have a good time with Bieber. "I would just get Justin Bieber all kinds of messed up," he said. "I just want to hang out with you, Justin."

Astin did note, however, they can keep the fun PG and age-appropriate. He shared, "You can drink water 'cause he's not legally allowed to drink yet."

Check out 7 exotic animals for Ke$ha's crazy, exotic life!

March 1 happens to be a big day for celebrity birthdays: The Biebs shares his born day with the always-glittering Ke$ha, who turned 26 on Friday. That news seemed to floor the trio.

"They said the world wasn't going to end," Teller joked. "You can't have all those things happening on one day."

Should the party-loving pop princess and Bieber decide to celebrate together, the guys couldn't help but use the moment to plug their comedy, out Friday (March 1).

"I think they say that when Justin Bieber and Ke$ha go to a movie together, and it's a college movie, you officially enter the fifth element," Astin offered. "That's like when the Mayans take over the world."

"Justin Bieber, listen up: Go see our movie, bro. Bieb it," Teller suggested. "Obviously, if you really belieb it, I'm sorry, if you really believe it, it's a really funny movie, and you should go check it out."

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