3. Porous membranes
Imagine a container that is separated into two equal compartments by a porous membrane - that is, by a very thin, flexible sheet with tiny holes in it. If one side is filled with water, and the other side filled with water and dissolved glucose, after a short time the amount of glucose compared to water will be the same in each side. Since there are fewer water molecules on the side with the glucose, fewer will make their way into the water-only side, while more from the water-only side will make their way into the side containing glucose. And since glucose initially is only on one side, the random movement of glucose molecules will soon equalise its distribution, on both sides of the membrane.