This object was made in Tinkercad. https://www.tinkercad.com/things/4qoeo4vyr6D
To make a spinner you need 12 M8 nuts and a 22mm bearing
These caps are designed to explain to children the principle of conservation of angular momentum, as seen in the picture and description below.
Conservation of Angular Momentum
The angular momentum of an isolated system remains constant in both magnitude and direction. The angular momentum is defined as the product of the moment of inertia I and the angular velocity. The angular momentum is a vector quantity and the vector sum of the angular momenta of the parts of an isolated system is constant. This puts a strong constraint on the types of rotational motions which can occur in an isolated system. If one part of the system is given an angular momentum in a given direction, then some other part or parts of the system must simultaneously be given exactly the same angular momentum in the opposite direction. As far as we can tell, conservation of angular momentum is an absolute symmetry of nature. That is, we do not know of anything in nature that violates it.