We know rice is a solid substance. Heat travels through solid substance by the process of conduction. By conduction heat from the fire heats up the pot and reaches the rice grains at bottom.
Then slowly heat transfers from one grain to another and after some time heat spreads to all places. But when we usually cook rice we also add water with rice. We know water is a liquid. In liquid heat travels by the process of convection, Water at the bottom of the pot heats up.
Hot water rises to the top and relatively cold water sinks to bottom. By this process heat spreads to all the parts. So, we can see while cooking heat travels by both conduction and convection.
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