This book, the first in a series of monographs written by "Captain Yo" Don Watson, is not intended for the casual yo-yo player. Instead, it's intended for engineers, designers, and physicistsinterested in the math behind the yo-yo. It provides would-be engineers with the equations useful in figuring out how to design a better yo-yo. This 38 page monograph deals with the Moment of Inertia and the Radius of Gyration. This is a specific-interest book to be sure, but a good book for that interest.