Water molecules can cross bilayer membranes with relative ease, 1 billion times more likely than these ions or these ions. So now, this is because they do not contain a full charge. There is a very high concentration of water outside and inside the cell. 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.