Their are many factors that make a yo-yo spin showing they are actually more complicated than people may first think. Gravity is an obvious force as it is what pulls the yo-yo down. Friction is the reason that it unwinds and returns. Without friction the yo-yo would not spin nor would it return to the starting position. When it is at this starting position it is containing the potential energy to fall and spin. This is then converted into kinetic energy when it starts its trip. The yo-yo has the most kinetic energy when it is at the bottom and all the potential energy has been converted. A major factor in a yo-yo is gyroscopic stability which is what keeps the yo-yo spinning in one direction. it's axis of rotation does not change and is always perpendicular to the axis of rotation.