Thermal convection is the physicists' favorite pattern forming system, because it enables us to create orderly structures under highly controlled conditions. Convection patterns arise from the circulation of hot and cold fluid, and they encompass such diverse phenomena as the appearance of wispy lines in oil as it is heated on the skillet and the granular structure of the sun's surface. Lord Rayleigh (in 1903) understood that the instability that causes convection is the result of a competition of forces. As the fluid near the bottom is heated, it expands and becomes lighter than the fluid above it. It rises, but dissipates its energy through contact with other fluid particles, becoming cooler and heavier. As the temperature of the bottom layer is increased, at some point the fluid at the bottom will be hot enough and light enough to rise to the top without dissipating all of its energy. This process of the hotter fluid rising and the cooler falling organizes itself into regular rolls. The critical temperature difference at which convection starts can be easily calculated for any fluid since it depends only on the depth of the layer and the known physical properties of the fluid.