Answer:
Objects become less dense than normal when heated. Hence when heating a fluid, the molecules closer to the heat source becomes hotter and their density reduces (buoyancy), hence, fewer hot molecules since they're denser, displace the buoyant molecules upwards. In so doing, the cooler molecules are now closer to the heat source which will make them also buoyant with time and they'll also be displaced upwards till the entire fluid is evenly heated.