Our algorithm is motivated by physical phenomena of a gas diffusing in a vessel. A particle of gas moves with constant speed until it meets a boundary of the vessel, then it reflects (the angle of incidence equals the angle of reflection) and so on. When the particle hits another one, its direction and speed changes. In our simplified model we assume that direction changes randomly while speed remains the same. Thus our model combines ideas of Hit-and-Run technique with the use of billiard trajectories.