Spiral Spinner – Hot air moves moves faster. Because the molecules now have the energy to move around more, they do and as they do they spread out and the gas becomes less dense. This lighter gas now rises up away from the heat source.
If a pinwheel is free to move and constructed correctly, the rising heat can now make it turn. The heat rises relatively straight up so to get the pinwheel to turn the blades need to be orientated in a way that causes the moving air to push them. As the hot air continues to move up it spins the pinwheel.