Conduction, Convection, Radiation

You will be able to see all three heat transfer processes occurring when you stand next to a bonfire.



You will probably need to stand at least 20 feet away from a big bonfire like the one shown in the picture. What keeps you away is heat radiating from the fire through infrared radiation. The flames and smoke are carried upward by convection: Air around the fire heats up and rises. The ground 3 feet beneath the fire will get hot, heated by conduction. The top layer of soil is directly heated (by radiation), and then the heat is conducted through layers of dirt deep into the ground.
To build a good thermos, what you want to do is reduce these three heat transfer phenomena as much as possible.