'Mom' Stars Anna Faris and Allison Janney Celebrate Their Birthdays with a Pie Fight!

It doesn’t matter how old you get, or how famous you are — the Happy Birthday song will always be 30 seconds of awkwardness to endure.

But if you're Anna Faris and Allison Janney, the stars of CBS's hit comedy Mom, you'll find a way to turn your birthday celebration into a fun-filled (and extremely messy!) memory.

Last week, the cast and crew of Mom, surprised their two leading ladies with a birthday cake and impromptu song at the live-taping of an upcoming episode, and only ETonline has your exclusive behind-the-scenes look at the hilariously candid moment.

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When Janney, who turned 55 on Nov. 19, and Faris, who turned 37 on Nov. 29., stepped out in front of their live studio audience, they were greeted with a gigantic chocolate cake, two pies, and a room full of excited fans.

After singing "Happy Birthday" to each other and successfully blowing out their candles, the stars decided to gift each other with a special present: a pie in the face! Take a look at our video above to witness all of the whipped-cream craziness for yourself.

Mom airs Thursdays at 8:30 p.m. on CBS.