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Take a pile of paper and place it on a flat, even surface. |
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With the flat of your finger, wind it around in circles. At first you don't think much is happening until eventually... |
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...the paper begins to unwind. Each piece of paper is slipping and gripping to the next, to create a beautiful fan shape. |
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The friction that would usually hold the pieces of paper together in a neat pile is broken by the movement of your finger. |
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And once you've gone that way, you can go the other. The pile starts to wind back into its original shape! |
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