While judging a performance, Simon suddenly turned to Demi to ask her opinion and the pop singer obviously wasn't expecting it. Her mouth was visibly full of mints!
So what's a girl to do? As you can see in our exclusive video, she took one of Simon's hands and spit them into it.
"Really, really grown up, Demi," Cowell growled. "You're an obnoxious little brat."
Hopefully she won't do the same on September 11, when The X Factor becomes the first TV show to ever host a red carpet premiere at the legendary Grauman's Chinese Theatre in Hollywood.
The judges have invited fans to come along too. For info go to: www.xfactortickets.com
All four of the judges, including Lovato, Britney Spears, L.A. Reid and Cowell, will be putting their handprints in cement outside the historical theater.
The X Factor premieres on Fox September 12.
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