I watched a Numberphile video on YouTube called "Wobbly Circles", so I decided to upload a model of what it would look like. If you don't feel like watching the video (I think you should, and you should also get the book mentioned), a circle rolls because it's center of mass remains constant. This system of 2 interlocked circles also rolls because it's center of mass is also constant. I'm not affiliated, just a fan.