A new study suggests that, for children with attention deficit hyperactivity disorder (ADHD), fidgeting and squirming is not a sign of distraction, but is a sign of mental effort and purpose. These behaviors appear to be directly linked to working memory processes—that is, the active ordering or updating of information in their minds:
http://mentalfloss.com/article/75962/fidgeting-may-help-kids-adhd-concentrate-study-finds
https://www.sciencedaily.com/releases/2016/02/160222111106.htm
http://jad.sagepub.com/content/early/2015/10/20/1087054715608439.abstract
And a great list of different fidgets:
