short stories - Children's sci-fi story about a group of British(?) school children who find an alien animal

I am looking for a children's novel published anytime from late 1950's through early 1980's. The story is about some British(?) schoolchildren (assuming ages 9 through 13 here) that find a strange animal and believe it is an alien.

The animal is small and round and fuzzy; about the size and shape of a bright yellow furry grapefruit. (Kind of reminds me of tribbles from Star Trek.) It has no apparent limbs, orifices, or sensory organs. The children are unsure what it is at first, but decide it must be an alien.

When a local scientist/professor slices into it as an experiment, the animal grows to the size of a beach ball. The local scientist/professor gets it from the children and tries to bring it to a university for further study.

That's about half way through the plot. I don't know how it ends, and don't have further knowledge of it.

Answer

I think that you are thinking of The Moonball, by Ursula Moray Williams. I remember the scene where the Moonball is cut open and grows tremendously in size.

Summary from Kirkus Reviews:

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.