For instance, we can look at sodium ions, chloride ions, potassium ions. We can look at calcium ions and so forth. Or we can look at molecules that don't have a charge yet they are very polar. For instance, sugar molecules. Sugar molecules are large enough and polar enough to not be able to pass across the membrane via simple diffusion. So in this case, if a molecule is polar and large, for example, sugars and sodium ions and so forth, they will not be able to simply dissolve inside that hydrophobic core.