And that means every time these collide, there's a likelihood that they're going to actually pass across. And number three is they are relatively small. So, for instance, if I compare, let's say, indol and water, the water is much more likely to actually pass across the cell membrane than indol because indol, even though it doesn't contain a full charge, indol is larger. We don't find indol in high concentrations and indol. When it collides, it cannot actually squeeze across that cell membrane because of its size and the fact that there are not too many collisions taking place between indo and the cell membrane in the first place. So three things that you have to consider to basically determine the permeability of a molecule across the cell membrane.