After Judy's class teases her about the limited potential of her favorite pets--guinea pigs--Judy discovers that her elderly pair has given birth to one with special talents. She trains "Jenius" to do several tricks, but he fails to perform at the pet show because of his well-learned fear of cats. Judy is vindicated, however, when Jenius does his best trick for her skeptical Dad. Amusing dialogue but predictable plot and cliched characters. An unusually slight effort from a fine author; still, an acceptable additional title for this reading level. Firmin's attractive full-color illustrations strike a good balance between realism and caricature.