The act of passing by osmosis through animal membranes, as in the distribution of poisons, gases, etc., through the body. Unlike absorption, diffusion may go on after death, that is, after the blood ceases to circulate.
molecules have energy, so when they are dissolved in water they move around a lot. The motion of any one atom or molecule has no particular direction, but a whole population of molecules will eventually get distributed more or less uniformly in the solution. Think about what happens if you put a drop of ink in a glass of water.