Knuckle cracking is a common behavior enjoyed by many. It can become a habit or a way to deal with nervous energy; some describe it as a way to "release tension." For some, it’s simply an annoying thing that other people do. --health.harvard.edu
cal. 195 (social impact)
Its essential feature is that it is annoying to others and presumably not harmful.
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a group creativity technique by which efforts are made to find a conclusion for a specific problem by gathering a list of ideas spontaneously contributed by its members --wikipedia