Generally useful, though a bit loose in my view. You said that the film grains are not touching each other, because we know that there is a lot of space between molecules -- a non sequitur. It also seems to me that in making a presentation such as this, designed for those fairly unfamiliar with film photography, it is an error to say "obviously," or to suggest that the viewer knows "instinctively" that faster films are grainier. In my view, these lapses contribute to confusion rather than clearing it up. I did think the flour and sugar illustration was somewhat useful heuristically, though perhaps a better explanation of the effect of those "left over" flour particles on sharpness might have improved comprehension for some. I realize that you didn't wish to get into sharpness, resolution, edge-effects, etc.
