I have a question regarding a cool toy I saw and I’m not sure how it works. It was a glass sphere, enclosed in the sphere was a rotating shaft with four plates. The plates had a shinny surface on one side and a dark surface on the other. The rotating shaft was free to move on each end and contained no motors or other electronics. The shaft rotates in the presence of strong sun light or the heat produced by the sunlight. I noticed when the sphere was shaded it did slow down. Shining an artificial light on it did cause it to rotate but very slowly. Can anyone explain to me the principle of operation. Thanks.