[quote]Then why dose the cold have a limit when heat can infinity become hotter?[/quote] Both have a limit, albeit theorethical. While 'absolute cold', zero Kelvin, would be achieved by completely stopping the Brownian motion, absolute heat is achieved by 'shortening' the wavelenght frequency to the planck distance (the shortest possible distance in the universe), which is calculated to be at around 141.000.000.000.000.000.000.000.000.000.000 K - luckily enough this temperature is simply called the planck temperature. While you could theoretically make more energy to make it hotter, anything beyond the planck temperature is simply beyond the concept temperature itself. It isn't a temperature anymore beyond this point and thus there is a hottest heat and coldest cold - it's just extremely unlikely that any of these extremes will ever be achieved in the history of this universe.