Copyright © 1996 by Paul O. Lewis
Imagine the particles are perfume molecules trapped inside a bottle. Remove the lid of the bottle by pressing the button and watch the perfume molecules diffuse throughout the room. The perfume molecules that escape just happen to be moving in a (random) direction that takes them out of the opening. No extra input of energy is needed to produce the net effect, which is the dispersion of perfume from an area of higher concentration (inside the box) to an area of lower concentration (outside the box). To reverse this process of diffusion (i.e. climb up the concentration gradient) would require an input of energy.
While this is a rather simplistic model of diffusion (well, ok, it is extremely simplistic), it does I think get across the main ideas in a way that is much more fun than reading the above paragraph.
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