What that means is they don't actually allow the movement of molecules that contain charge or ions. They only allow the movement of water molecules. So they're water channels that allow the movement of molecules, water molecules down their concentration gradient when the cell requires it. So from a high to a low concentration. So when would our cells of the body need to basically take the water and move it to the other side of the cell membrane very quickly? Well, one example is when we produce tears.