when you heat water in the saucepan.............the water molecules at the very bottom of the saucepan gets heated first but the water molecules on the top of the saucepan are still not heated..................when the water molecules are heatd then, the heated water molecules go upwards.....to fill the space of the heated water molecules that went upwards, the cold water molecules from the top of the saucepan come downwards ........this process keeps on repeating until all the water in the saucepan is fully heated
the picture below best describes this phenomenon
