If two media at different temperatures contact each other, they are exchanging energy. In thermodynamics, this energy transfer is called heat transfer. Actually direct energy exchange is realizing at the collisions of atoms and molecules of the hotter media with atoms and molecules of the cooler media. The simulation shows a gas confined in a closed cylinder in time when a partition separating two parts of different temperature has been removed. One half of molecules have the energy greater then another half. The hot red molecules and the blue cold molecules collide with chaotic character and exchange the energy. Run the simulation and observe the changing (on graphics) of kinetic energy of the two systems in the process of molecular collisions.