For a convection cell to be set up in a fluid, it must be heated from below and be able to radiate heat from the top. Convection cells occur when a particular packet of fluid becomes trapped in a cycle of heating and cooling, causing it to rise and fall.
Fluids become less dense when heated, so hotter fluids are lighter per unit volume than the same fluids when colder. Because fluids can flow, a fluid heated from below tends to flow upward, away from the heat, displacing any colder fluid above it. Because the hotter fluid is rising, the colder fluid must be displaced downward, bringing it nearer to the source of heat. At the same time, the hotter fluid, away from the heat source, cools as it radiates heat away into the environment.
Convection cells can easily form within open containers, such as a pot on a stove. They form in other conditions as well, however, at vastly varying scales. Large storm clouds are examples of convection cells, where warm air rises from the Earth's surface through the center of the cloud and cold air falls along the outside. Convection cells also cause what is known as granulation on the surface of the sun.