Whenever a fluid bulk is heated form the lower layers then due to the variation of the density of the fluid at different temperature we observe the movement of molecules leading to convection.
When the lowest level of the fluid is heated it gains temperature and the molecular bulk expands on heating and its density becomes low with respect to the bulk fluid around it and hence it flows upwards to the top most layer being lighter in weight and the lowest layer is occupied by the subsequent colder and denser layer.
Then again the lowest layer is heated and the process continues forming a cycle heating through the bulk transfer of fluid layers called convention.
In the process of convection, first the molecules of substance gets heat and becomes heated. As the molecules heated, their density decreases and hence they rises up. And the molecules which are cooler, they fall and comes in contact of higher temperature and then they rises up. the process repeats itself until the entire liquid heated uniformly.
