Convection is the circular moment of heat through a fluid such as air or water. The moment up guys because the utility causes the flood molecules to spread out with an excited state of energy.

This surprising the cause targeted forward to become less dense than the surrounding fluid, so it rises. As the energy dissipates it gains density and again starts to settle back down.

Activity:

So, in your example, the flame at the bottom of the oven heats the surrounding air which circulates inside the oven and transfers energy to the food. Once the heat hits the food, the inside of it gets cooked by conduction.