There creatures have crystals throughout there bodies, and like to get crystals from wherever they can; what they do with those crystals, is mold them in to balls that many would call marbles, and that they will come up with ways to entertainment each other with a those crystals marbles balls! Most of those games are what we would call marble based, they will also just try using them as normal balls. These creatures are incredibly strong for there small size.
(Idea by Jacqal)