This is an updated version of a past demo. Cell membranes are selectively permeable. That is the membrane allows some materials to pass through and not others. This allows the cell to maintain homeostasis, a condition in which the cell maintains the proper levels of needed materials. In this demo the plastic of the bag acts as a selectively permeable membrane. Iodine is small enough to pass through the plastic, while the larger water molecules are too big. Watch the video on how to do it and to see what happens.