Acho que você está pensando em The Moonball , de Ursula Moray Williams . Eu me lembro da cena em que a Moonball é aberta e cresce tremendamente em tamanho.
Resumo de Críticas de Kirkus :
A round hairy object--no legs, no ears, no tail, no eyes, not even a heart beating--appears on the cricket field after a sudden, terrifying storm, and the children dub it moonball--a useful appellation which is part identification, part imagination. But moonball is more than it first appears--it is the minimum animal, sensibility objectified. In their efforts to save it from an overbearing, fatuous scientist, the children discover its "strange, funny ways, odd endearments, its comfort and its beauty." They are happier, less fractious, and one boy, Freddie the Nipper, "a kind of Huckleberry Finn," is relieved of his rebellious stance and reunited with his father. As symbol, moonball is the familiar magic stone or magic key; but as object it is touching in its tenderness, awful in its fury, hilarious in its calculated contrariness. The quick characterization of individuals minimizes identification and the deep-rooted descriptive style demands close attention, but the substance will sustain the serious reader who can be persuaded to try a book that is bigger than its size.