Why do you mix oil and petrol in a 2 stroke?

Two-stroke engines usually do not have an internal lubrication system; the moving parts are lubricated by a small flow of lubricating oil which enters the combustion chamber pre-mixed with the fuel. This oil is usually not completely burned during the power stroke, and therefore a portion of it clings to the cylinder walls, passes into the crankcase, and eventually reaches all moving parts of the engine.