Britney Spears and Demi Lovato have become partners in crime since officially joining The X Factor in May.

At the Kansas City, Missouri auditions June 8, X Factor UK judge Louis Walsh, 59, filled in for an ill Simon Cowell, 52, and taught the two pop stars a funny game he likes to play on the European version of the reality hit, Us Weekly has learned.

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According to a source, the judges secretly chose a word before each contestant performed and then tried to use it during their critiques. The more difficult the word, the better. "Every time one of them said the word, they all started giggling like naughty school children."

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Spears started the game by choosing the word "tiger." Judge L.A. Reid, 56, told one contestant "he looks like a tiger in his tank," which a giggling Lovato followed up by saying, "I can see a little tiger action."

Spears was next to chime in, saying "he was like a big tiger." She and Lovato had trouble keeping a straight face and were laughing uncontrollably, per the source.

The mischief continued when Lovato chose the word "umbrella." Walsh told the contestant, "You remind me of Rihanna -- that 'Umbrella' girl." Lovato then told her, "You're the umbrella to my rain," only to be one-upped by Spears, who said, "You're my umbrella to everything."

The judges also chose the words "slinky," "punk rock," "caution" and "mountain."

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"Demi, Britney and L.A. enjoyed the game, but no one is sure if Simon will be too happy with Louis for leading everyone astray," a set source tells Us. "He has been doing the X Factor in the UK for nine years and everyone loves him, but he's always making trouble!"

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If anything, it gave "Criminal" singer Spears, 30, and "Give Your Heart a Break" chart topper Lovato, 19, a chance to put rumors of backstage feuds to rest.

"Britney and Demi are getting along great," an insider tells Us. "Whenever they aren't filming, Demi and Britney are chatting and hanging out. . . They are seriously bonding big time. It's nice to see."

This article originally appeared on How Britney Spears, Demi Lovato Are Bonding at X Factor