That's the slightly more difficult a question to actually answer. So, for instance, let's compare sodium and potassium. Now, Potassium Dionic radius of potassium is about 1.33 angstromes, while the ionic radius of sodium is about 0.95. And so we see that the ionic radius of these potassium ions is about 40% greater than the ionic radius of our sodium ions. Let's suppose this one is sodium. So if these sodium ions are so much smaller, the question is, why cannot that sodium ion actually pass across that membrane?