Celebrities attending Saturday's 25th annual Kids' Choices Awards should start getting nervous. Really nervous. While awards are presented during the kid-friendly Nickelodeon show, fans really just want to see the slime. From Mary-Kate and Ashley Olsen's double slime to Katy Perry's shocking green surprise, it's always fun to see stars -- especially those always looking their best -- get covered in that bright, sticky, Nickelodeon-signature goo. We've rounded up some of our favorite KCA slime...