In statistical physics we have for an ideal gas about the average kinetic energy for its molecules: Ekin=½kT Now in my book this is derived using the ideal gas law as an experimental fact, but that does not really help you get a deeper understanding, does it? I'm assuming that this can be derived from statistical mechanics. I wonna ask the following? What is the intuition behind, that the speed of the molecules is bigger the less they weigh? This follows from the fact that every molecule regardless of mass, apparantly on average have the same kinetic energy.