I am thinking that convection current does happens not because of changed in density. I based this argument with viewing the problem from particle / molecular scale. Density = Mass/ Volume. Yet in molecules of water, discussing mass and volume would less likely to be relevant as mass and volume of each molecule is identical. Therefore, I prefer not to understad convection current as incerease and decrease in density due heat. I prefer to see it as particles having more or less energy (calculated from the movement speed) moving in brownian random motion. What happened actually is that the heated particle contain more energy and move faster (randomly). Their horizontal motion (XY plane) are perfectly disturbed by inter-molecular collision within themselves and walls of the container while their vertical motion (Z axis) are affected by the same factor and gravity. This gravity factor enable particle with more energy move higher then the particle with lesser. This action is simulated in www.geocities.com/arian_m3/heat.swf by using macromedia flash. I am assuming that this effect caused the change in density and therefore convection current not just by heatting a volume of fluid which then rise. In the animation, red is the hotter particle and blue is the cooler particle. The system is not heatted and the hot stays hot while the cold stays cold. I am writing an essay about this topic and waiting for suggestion and comments.