It Came from Beneath the Sink! is a novella in the series, Goosebumps by R. L. Stine.
Two siblings who just moved in, discovers a sponge living in their sink that feeds on pain and suffering. When their friend brings his "Encyclopedia of the Weird", they find it is called a Grool. From the book Katrina learns that by having it in her possession, it causes bad luck, often resulting in injury, both to her and to those around her. Also, she learns that if the Grool's "owner" gives it to someone else, they die. After many failed attempts to dispose of it, she learns that it can not be destroyed in any violent or physical way. After brainstorming, she realizes how to destroy it by being nice to it. Katrina kisses the Grool and it shrinks down to the size of an ordinary sponge. Since the Grool can't be thrown away, Katrina keeps the Grool as a pet. The book ends with Killer (Katrina's dog) coming home after being missing (thanks to the Grool's curse) and bringing Katrina a Lanx (a creature mentioned in the "Encyclopedia of the Weird" that is shaped like a potato, but has fangs like a vampire).
Kat and her brother, Daniel, are so lucky. They just moved to a new house with tons of rooms, two balconies, and a lawn the size of a football field! But all that good luck is about to run out. Because there's something really evil living in their new house. Something that's moving. Watching. Waiting. Something that comes from beneath the kitchen sink...
The Grool (according to Goosebumps) is a mythical sponge-like being which feeds off bad luck. It is considered a bad luck charm due to this reason. It changes colors when happy, and cannot be destroyed by force or strength (attempting to do so will only make the bad luck worse), and if passed on to another person, the previous owner will die within 24 hours unless they get it back before then. It can only be destroyed by showing it love and compassion. Any lingering effects of the Grool apparently wear off after it is destroyed.
A cousin of the Grool is the Lanx, which appears to be a potato that acts like a vampire. After latching onto a person with these teeth, it feeds off their energy and will not let go until it has drained them completely.
In the TV series they find out what the Grool is in the "Encyclopedia of the Weird".
It's warm! It's breathing! And it doesn't do dishes!
This episode starred a young Katharine Isabelle
as Katrina 'Kat' Merton.