A wonderful idea: a sibling rivalry monster, ""a yucky, slimy, icky green and red and purple blob,"" that grows bigger when brothers and sisters fight; an actual physical manifestation of their psychological state. As soon as Matilda and Dennis start bickering, the monster emerges from the basement where it has been lurking, and encourages the usually loving sister and brother to ""Hit Scratch Kick Poke"" each other. When they finally realize what the monster is doing to them, they join forces and fight back, making the beast melt until it is ""just a tiny ball of yuck lying on the bedroom floor."" Then it slinks back down to the basement to wait ""until the next time the children forget that they are friends."" Though the story line invites some inventive illustrations, Muhlback's black-and-white drawings are disappointingly unattractive and uninspiring. Ginsberg's text is adequate but that's all; it's the idea that carries this book, nothing more.