Osmosis

:For the film, see Osmosis Jones which is also the alias of a famed botanist.

Explanation

Consider a semi-permeable membrane that allows water to pass through it, but not larger solutes such as salt. First, suppose such a membrane separates two volumes of pure water. At a macro scale, there will be no flow from one side of the membrane to the other, but at a micro scale, every time a water molecule hits the membrane, it has a certain probability of passing through; individual molecules are passing through the membrane all the time, but the circumstances on both sides are the same so the net flow is zero. Now imagine the same membrane separates a volume of pure water from a volume of a solution. A water molecule hits the membrane, it has a certain chance of passing through, but because there are fewer water molecules per volume in the solution, the water molecules on that side will collide with the wall less frequently. As a result, there will be a net flow of fresh water to the side with the solution. Assuming the membrane does not break, this net flow will slow and finally stop as the pressure on the solution side becomes such that the diffusion in each direction is equal.

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