The story follows Kat Merton, who has just moved to a new house with her family. The house is huge, and Kat couldn't be more excited to live there. But upon arrival, she finds something under the kitchen sink -- a sponge. But wait...sponges don't have mouths. And they don't laugh. Dogs don't growl at them. And they certainly don't cause bad luck.
But this sponge does. After several incidents that include her dad falling off a ladder and her birthday party being cancelled due to rain, Kat does some research and finds out that her personified sponge is actually a Grool, a creature that causes bad luck and feeds off of it. The more bad things that happen to Kat, the more angry she gets. And the more angry she gets, the happier the Grool becomes.
Kat is stuck in a vicious cycle for the rest of the book. She tries everything she can to rid herself of the Grool, but it keeps coming back and shows no signs of giving her a break. Even worse, Kat learns that if she gives the Grool up, she'll die. So it seems like Kat is stuck with bad luck for the rest of her life.
Except, you know, she's not. Finally, in front of her bewildered brother and his friend, she starts to pet the Grool and sing to it. She coos about how much she loves it and even kisses it. Slowly, the Grool starts to shrivel up until it's nothing but a few flakes of what used to be a sponge-like creature (in more ways than one).
Sounds pretty ridiculous, of course, but we can learn a lot from Kat Merton and the evil sponge. Humans are sort of like Grools; we feed off negativity, and seeing people in pain sometimes makes us happy -- especially if they've done something to wrong us. If we continue to treat them the way they're treating us, we'll be stuck in that cycle just like Kat. But if we treat them the opposite of what they're expecting, without the misery they feed off of, we've broken the cycle.
It's extremely hard to do sometimes. It takes a huge hit to our already fragile egos when we try to be nice to someone who's done nothing but make us feel like crap. But it's not just a noble thing to do -- it can keep you both from experiencing more unnecessary pain. Sometimes if you want to change things for the better, you have to get outside of your comfort zone and even bruise your ego a bit.
And, you know, don't pick up any laughing sponges.
Except maybe this one. (source)