We see that for a molecule to actually dissolve across that cell membrane, it must be able to interact in a stabilizing way with those tails, with the hydrocarbon tails of those phospholipid molecules. And so non polar molecules can dissolve across the membrane, but polar molecules cannot. Now, technically, that is not actually the entire story because we see water molecules are actually an exception to that rule. Water molecules can actually pass across the cell membrane. In fact, water molecules are 1 billion times more likely to actually pass across the cell membrane than potassium or than potassium or sodium ions. Now, water, as we know, is in fact a polar molecule.