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? How is it that the potassium ion channel actually blocks the movement of this smaller sodium ion? Well, the answer lies in this salvation cage. So we saw that in the case for potassium, when potassium loses this salvation cage, that requires an input of energy.