The heated cooking oil on the bottom is less dense and wants to rise. The top of the oil cools down by contact with the air and sinks again. This motion becomes circular and creates rolls of liquid, which self-organise into a honeycomb pattern which can be easily observed. Quite a bit of research has been carried out on this phenomenon, which anyone can create in the kitchen and we now have an explanation as to why the pattern of the convection cells is honeycomb. The form of the convection rolls depends on the shape of the container in which the liquid is heated.
Hexagonal patterns seem to develop easily in round pans.
Other containers may lead to long, rectangular rolls, with a square cross section. It is interesting that many parameters such as the unit size of the convection cell are determined, whereas the direction of the circular motion is undetermined at the onset of convection. When the temperature gradient within the oil layer has built up to a certain critical value, each of the many scattered convection currents present in the oil finds that they conserve energy better if they share their region of descending flow, with the down-flow regions of their immediate neighbours. This stops any contraflow problems. This cooperative repositioning of the centres of convection forms a regular pattern of closely packed convection cells.