Get a round cardboard tube, such as people use for sending things through the post which they do not wish they do not wish to have ceased, and cut an inch off it, either with a fretsaw or a pocket-knife. Be careful that in doing this you do not flatten any part of the circumference, because it must be a perfect ring. Then stick a big dab of plasticene on it, as shown in the diagram. Now stand the ring on a slightly inclined plane and it will roll slowly uphill, thus defying gravity. The dab of material, stuck on the inner face of the ring, must be placed at the top, just a triffle towards the uphill direction, when starting.