And what this says is that the lighter the molecule, the faster its rate or the higher its rate. Now we're going to talk about something called diffusion. Diffusion is the movement or the flux of one type of gas molecule into another gas or an empty space. And now we can use grammar's law to approximate the rates of diffusion. Now, the reason that we approximate is because of something called mean free path. Now, the average or the mean free path of a gas molecule is the distance it travels between any two collisions.