And three, these bilayer membranes have the propensity to fix themselves. Why? Well, let's suppose some type of hole is formed in that bimolecular sheet, in the lipid bilayer. Once that hole is formed, we go back to this picture where the water molecules become trapped around that hydrophobic tail and that is not a favorable system. And so what will happen is that hole that is formed will be spontaneously fixed to basically decrease the energy of that system. And that's exactly what we mean by the bilayer membrane has the spontaneous ability, the propensity, to actually fix itself.