Imagine a cliff with lots of ledges in lots of configurations: large, small, flat, tilted, cracked. Then imagine lots of rocks falling from the top, down the side of the mountain--they, too, are in every possible configuration: large, small, flat, round.
What will happen over time?
Small rocks will fall in cracks, flat rocks may land on ledges and stay there. Round rocks will probably bounce and roll to the bottom. After a while, you'll get very clear patterns and distinctions--stacks of flat rocks on certain ledges, while other ledges are distinctly rock-free. In some cracks you'll find small rocks, while in others, only sand; and in any given crack, you'll never find a rock larger than a certain amount in any direction (the amount being the widest spot in the opening of the crack). Round rocks will form unstable piles at the bottom.
There are patterns, trends and seeming progressions. It may even look like the rocks are being deliberately organized. After all, no matter how long you stare at a bunch of rocks, they never just up and stack themselves. And how would the rocks know, anyway, whether they are round, and thus belong at the bottom; or small, and thus belong in a crack; or flat, and thus belong on a ledge?
The point is, when everything happens, in every possible configuration, the stuff that is stable will stick around and the stuff that is unstable will disappear.
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