Hoberman Sphere

A Hoberman sphere is used to demonstrate conservation of angular momentum (L = Iω). The sphere is suspended from a fixed point above and is free to rotate about its vertical axis. Pulling the string attached to the bottom causes the sphere to contract, reducing its radius and therefore reducing its moment of inertia, which gives the sphere a much larger angular velocity.

This is how figure skaters increase their angular velocity -- by bringing their arms and non-plant leg in closer to their axis of rotation while spinning.